diff --git a/b2sdk/utils/thread_pool.py b/b2sdk/utils/thread_pool.py
index f07c11ccb..b9a190c89 100644
--- a/b2sdk/utils/thread_pool.py
+++ b/b2sdk/utils/thread_pool.py
@@ -9,27 +9,106 @@
 ######################################################################
 from __future__ import annotations
 
-from concurrent.futures import ThreadPoolExecutor
+import os
+from concurrent.futures import Future, ThreadPoolExecutor
+from typing import Callable
+
+try:
+    from typing_extensions import Protocol
+except ImportError:
+    from typing import Protocol
 
 from b2sdk.utils import B2TraceMetaAbstract
 
 
+class DynamicThreadPoolExecutorProtocol(Protocol):
+    def submit(self, fn: Callable, *args, **kwargs) -> Future:
+        ...
+
+    def set_size(self, max_workers: int) -> None:
+        """Set the size of the thread pool."""
+
+    def get_size(self) -> int:
+        """Return the current size of the thread pool."""
+
+
+class LazyThreadPool:
+    """
+    Lazily initialized thread pool.
+    """
+
+    _THREAD_POOL_FACTORY = ThreadPoolExecutor
+
+    def __init__(self, max_workers: int | None = None, **kwargs):
+        if max_workers is None:
+            max_workers = min(
+                32, (os.cpu_count() or 1) + 4
+            )  # same default as in ThreadPoolExecutor
+        self._max_workers = max_workers
+        self._thread_pool: ThreadPoolExecutor | None = None
+        super().__init__(**kwargs)
+
+    def submit(self, fn: Callable, *args, **kwargs) -> Future:
+        if self._thread_pool is None:
+            self._thread_pool = self._THREAD_POOL_FACTORY(self._max_workers)
+        return self._thread_pool.submit(fn, *args, **kwargs)
+
+    def set_size(self, max_workers: int) -> None:
+        """
+        Set the size of the thread pool.
+
+        This operation will block until all tasks in the current thread pool are completed.
+
+        :param max_workers: New size of the thread pool
+        :return: None
+        """
+        if self._max_workers == max_workers:
+            return
+        old_thread_pool = self._thread_pool
+        self._thread_pool = self._THREAD_POOL_FACTORY(max_workers=max_workers)
+        if old_thread_pool is not None:
+            old_thread_pool.shutdown(wait=True)
+        self._max_workers = max_workers
+
+    def get_size(self) -> int:
+        """Return the current size of the thread pool."""
+        return self._max_workers
+
+
 class ThreadPoolMixin(metaclass=B2TraceMetaAbstract):
     """
     Mixin class with ThreadPoolExecutor.
     """
-    DEFAULT_THREAD_POOL_CLASS = staticmethod(ThreadPoolExecutor)
+
+    DEFAULT_THREAD_POOL_CLASS = LazyThreadPool
 
     def __init__(
         self,
-        thread_pool: ThreadPoolExecutor | None = None,
+        thread_pool: DynamicThreadPoolExecutorProtocol | None = None,
         max_workers: int | None = None,
-        **kwargs
+        **kwargs,
     ):
         """
         :param thread_pool: thread pool to be used
         :param max_workers: maximum number of worker threads (ignored if thread_pool is not None)
         """
-        self._thread_pool = thread_pool if thread_pool is not None \
-            else self.DEFAULT_THREAD_POOL_CLASS(max_workers=max_workers)
+        self._thread_pool = (
+            thread_pool
+            if thread_pool is not None else self.DEFAULT_THREAD_POOL_CLASS(max_workers=max_workers)
+        )
+        self._max_workers = max_workers
         super().__init__(**kwargs)
+
+    def set_thread_pool_size(self, max_workers: int) -> None:
+        """
+        Set the size of the thread pool.
+
+        This operation will block until all tasks in the current thread pool are completed.
+
+        :param max_workers: New size of the thread pool
+        :return: None
+        """
+        return self._thread_pool.set_size(max_workers)
+
+    def get_thread_pool_size(self) -> int:
+        return self._thread_pool.get_size()
diff --git a/b2sdk/v2/transfer.py b/b2sdk/v2/transfer.py
index da4d28b68..151f5ff1c 100644
--- a/b2sdk/v2/transfer.py
+++ b/b2sdk/v2/transfer.py
@@ -9,46 +9,17 @@
 ######################################################################
 from __future__ import annotations
 
-from concurrent.futures import Future, ThreadPoolExecutor
-from typing import Callable
-
 from b2sdk import _v3 as v3
-
-
-class LazyThreadPool:
-    """
-    Lazily initialized thread pool.
-    """
-
-    def __init__(self, max_workers: int | None = None, **kwargs):
-        self._max_workers = max_workers
-        self._thread_pool = None  # type: 'Optional[ThreadPoolExecutor]'
-        super().__init__(**kwargs)
-
-    def submit(self, fn: Callable, *args, **kwargs) -> Future:
-        if self._thread_pool is None:
-            self._thread_pool = ThreadPoolExecutor(self._max_workers)
-        return self._thread_pool.submit(fn, *args, **kwargs)
-
-    def set_size(self, max_workers: int) -> None:
-        if self._max_workers == max_workers:
-            return
-        if self._thread_pool is not None:
-            raise RuntimeError('Thread pool already created')
-        self._max_workers = max_workers
+from b2sdk.utils.thread_pool import LazyThreadPool  # noqa: F401
 
 
 class ThreadPoolMixin(v3.ThreadPoolMixin):
-    DEFAULT_THREAD_POOL_CLASS = staticmethod(LazyThreadPool)
-
-    # This method is used in CLI even though it doesn't belong to the public API
-    def set_thread_pool_size(self, max_workers: int) -> None:
-        self._thread_pool.set_size(max_workers)
+    pass
 
 
-class DownloadManager(v3.DownloadManager, ThreadPoolMixin):
+class DownloadManager(v3.DownloadManager):
     pass
 
 
-class UploadManager(v3.UploadManager, ThreadPoolMixin):
+class UploadManager(v3.UploadManager):
     pass
diff --git a/changelog.d/+set_threads.added.md b/changelog.d/+set_threads.added.md
new file mode 100644
index 000000000..45ec116cf
--- /dev/null
+++ b/changelog.d/+set_threads.added.md
@@ -0,0 +1 @@
+Add `set_thread_pool_size`, `get_thread_pool_size` to *Manger classes.
diff --git a/test/unit/utils/test_thread_pool.py b/test/unit/utils/test_thread_pool.py
new file mode 100644
index 000000000..5742d3e35
--- /dev/null
+++ b/test/unit/utils/test_thread_pool.py
@@ -0,0 +1,43 @@
+######################################################################
+#
+# File: test/unit/utils/test_thread_pool.py
+#
+# Copyright 2024 Backblaze Inc. All Rights Reserved.
+#
+# License https://www.backblaze.com/using_b2_code.html
+#
+######################################################################
+from concurrent.futures import Future
+
+import pytest
+
+from b2sdk.utils.thread_pool import LazyThreadPool
+
+
+class TestLazyThreadPool:
+    @pytest.fixture
+    def thread_pool(self):
+        return LazyThreadPool()
+
+    def test_submit(self, thread_pool):
+
+        future = thread_pool.submit(sum, (1, 2))
+        assert isinstance(future, Future)
+        assert future.result() == 3
+
+    def test_set_size(self, thread_pool):
+        thread_pool.set_size(10)
+        assert thread_pool.get_size() == 10
+
+    def test_get_size(self, thread_pool):
+        assert thread_pool.get_size() > 0
+
+    def test_set_size__after_submit(self, thread_pool):
+        future = thread_pool.submit(sum, (1, 2))
+
+        thread_pool.set_size(7)
+        assert thread_pool.get_size() == 7
+
+        assert future.result() == 3
+
+        assert thread_pool.submit(sum, (1,)).result() == 1
diff --git a/test/unit/v2/test_transfer.py b/test/unit/v_all/test_transfer.py
similarity index 56%
rename from test/unit/v2/test_transfer.py
rename to test/unit/v_all/test_transfer.py
index 9636b199e..584bf6d68 100644
--- a/test/unit/v2/test_transfer.py
+++ b/test/unit/v_all/test_transfer.py
@@ -1,6 +1,6 @@
 ######################################################################
 #
-# File: test/unit/v2/test_transfer.py
+# File: test/unit/v_all/test_transfer.py
 #
 # Copyright 2022 Backblaze Inc. All Rights Reserved.
 #
@@ -11,19 +11,26 @@
 
 from unittest.mock import Mock
 
+from apiver_deps import DownloadManager, UploadManager
+
 from ..test_base import TestBase
-from .apiver.apiver_deps import DownloadManager, UploadManager
 
 
 class TestDownloadManager(TestBase):
     def test_set_thread_pool_size(self) -> None:
         download_manager = DownloadManager(services=Mock())
-        download_manager.set_thread_pool_size(21)
-        self.assertEqual(download_manager._thread_pool._max_workers, 21)
+        assert download_manager.get_thread_pool_size() > 0
+
+        pool_size = 21
+        download_manager.set_thread_pool_size(pool_size)
+        assert download_manager.get_thread_pool_size() == pool_size
 
 
 class TestUploadManager(TestBase):
     def test_set_thread_pool_size(self) -> None:
         upload_manager = UploadManager(services=Mock())
-        upload_manager.set_thread_pool_size(37)
-        self.assertEqual(upload_manager._thread_pool._max_workers, 37)
+        assert upload_manager.get_thread_pool_size() > 0
+
+        pool_size = 37
+        upload_manager.set_thread_pool_size(pool_size)
+        assert upload_manager.get_thread_pool_size() == pool_size