diff --git a/content/browser/renderer_host/back_forward_cache_impl.h b/content/browser/renderer_host/back_forward_cache_impl.h
index 0a57cf725d4830..6bbf42b299e6ec 100644
--- a/content/browser/renderer_host/back_forward_cache_impl.h
+++ b/content/browser/renderer_host/back_forward_cache_impl.h
@@ -11,6 +11,7 @@
 #include <unordered_set>
 
 #include "base/feature_list.h"
+#include "base/gtest_prod_util.h"
 #include "base/memory/raw_ptr.h"
 #include "base/memory/raw_ref.h"
 #include "base/memory/weak_ptr.h"
@@ -104,6 +105,28 @@ struct CONTENT_EXPORT BackForwardCacheCanStoreDocumentResultWithTree {
 // frozen state and is kept in this object. They can potentially be reused
 // after an history navigation. Reusing a document means swapping it back with
 // the current_frame_host.
+//
+//
+// BackForwardCache Size & Pruning Logic:
+//
+// 1. `EnforceCacheSizeLimit()` is called to prune the cache size down on
+//    storing a new cache entry, or when the renderer process's
+//    `IsProcessBackgrounded()` state changes.
+//    A. [Android-only] The number of entries where `HasForegroundedProcess()`
+//       is true is pruned to `GetForegroundedEntriesCacheSize()`.
+//    B. Prunes to `GetCacheSize()` entries no matter what kinds of tabs
+//       BackForwardCache is in.
+//    C. When a `RenderFrameHost` enters BackForwardCache, it schedules a task
+//       in `RenderFrameHostImpl::StartBackForwardCacheEvictionTimer()` to
+//       evicts the outermost frame after
+//       `kDefaultTimeToLiveInBackForwardCacheInSeconds` seconds.
+// 2. In `performance_manager::policies::BFCachePolicy`:
+//    A. (To Launch) [Desktop-only] On moderate memory pressure, the number of
+//       entries in a visible tab's cache is pruned to
+//       `ForegroundCacheSizeOnModeratePressure()`. The number in a non-visible
+//       tab is pruned to `BackgroundCacheSizeOnModeratePressure()`.
+//    B. (To Launch) [Desktop-only] On critical memory pressure, the cache is
+//       cleared.
 class CONTENT_EXPORT BackForwardCacheImpl
     : public BackForwardCache,
       public RenderProcessHostInternalObserver,
@@ -361,8 +384,6 @@ class CONTENT_EXPORT BackForwardCacheImpl
       BackForwardCacheCanStoreDocumentResult& eviction_reason);
 
  private:
-  FRIEND_TEST_ALL_PREFIXES(BackForwardCacheMetricsTest, AllFeaturesCovered);
-
   // Destroys all evicted frames in the BackForwardCache.
   void DestroyEvictedFrames();
 
@@ -551,6 +572,13 @@ class CONTENT_EXPORT BackForwardCacheImpl
   };
 
   base::WeakPtrFactory<BackForwardCacheImpl> weak_factory_;
+
+  // For testing:
+  FRIEND_TEST_ALL_PREFIXES(BackForwardCacheMetricsTest, AllFeaturesCovered);
+  FRIEND_TEST_ALL_PREFIXES(BackForwardCacheActiveSizeTest, ActiveCacheSize);
+  FRIEND_TEST_ALL_PREFIXES(BackForwardCacheOverwriteSizeTest,
+                           OverwrittenCacheSize);
+  FRIEND_TEST_ALL_PREFIXES(BackForwardCacheDefaultSizeTest, DefaultCacheSize);
 };
 
 // Allow external code to be notified when back-forward cache is disabled for a
diff --git a/content/browser/renderer_host/back_forward_cache_impl_unittest.cc b/content/browser/renderer_host/back_forward_cache_impl_unittest.cc
index 7e2d8bbe447667..4005ba0daf8991 100644
--- a/content/browser/renderer_host/back_forward_cache_impl_unittest.cc
+++ b/content/browser/renderer_host/back_forward_cache_impl_unittest.cc
@@ -3,7 +3,13 @@
 // found in the LICENSE file.
 
 #include "content/browser/renderer_host/back_forward_cache_impl.h"
+
+#include "base/test/scoped_feature_list.h"
+#include "content/test/test_render_frame_host.h"
 #include "content/test/test_render_view_host.h"
+#include "testing/gmock/include/gmock/gmock.h"
+#include "testing/gtest/include/gtest/gtest.h"
+#include "third_party/blink/public/common/scheduler/web_scheduler_tracked_feature.h"
 
 namespace content {
 
@@ -88,4 +94,60 @@ TEST_F(BackForwardCacheImplTest, SecondCrossOriginReachable) {
             blink::mojom::BFCacheBlocked::kNo);
 }
 
-}  // namespace content
\ No newline at end of file
+// Covers BackForwardCache's cache size-related values used in Stable.
+// See docs/back_forward_cache_size.md for more details.
+class BackForwardCacheActiveSizeTest : public ::testing::Test {
+ protected:
+  void SetUp() override {
+    feature_list_.InitWithFeaturesAndParameters(
+        {{features::kBackForwardCache,
+          {{"cache_size", "6"}, {"foreground_cache_size", "2"}}}},
+        {});
+  }
+
+ private:
+  base::test::ScopedFeatureList feature_list_;
+};
+
+TEST_F(BackForwardCacheActiveSizeTest, ActiveCacheSize) {
+  EXPECT_EQ(BackForwardCacheImpl::GetCacheSize(), 6u);
+  EXPECT_EQ(BackForwardCacheImpl::GetForegroundedEntriesCacheSize(), 2u);
+  EXPECT_TRUE(BackForwardCacheImpl::UsingForegroundBackgroundCacheSizeLimit());
+}
+
+// Covers overwriting BackForwardCache's cache size-related values.
+// When "cache_size" or "foreground_cache_size" presents in both
+// `kBackForwardCacheSize` and `features::kBackForwardCache`, the former should
+// take precedence.
+class BackForwardCacheOverwriteSizeTest : public ::testing::Test {
+ protected:
+  void SetUp() override {
+    feature_list_.InitWithFeaturesAndParameters(
+        {{kBackForwardCacheSize,
+          {{"cache_size", "8"}, {"foreground_cache_size", "4"}}},
+         {features::kBackForwardCache,
+          {{"cache_size", "6"}, {"foreground_cache_size", "2"}}}},
+        {});
+  }
+
+ private:
+  base::test::ScopedFeatureList feature_list_;
+};
+
+TEST_F(BackForwardCacheOverwriteSizeTest, OverwrittenCacheSize) {
+  EXPECT_EQ(BackForwardCacheImpl::GetCacheSize(), 8u);
+  EXPECT_EQ(BackForwardCacheImpl::GetForegroundedEntriesCacheSize(), 4u);
+  EXPECT_TRUE(BackForwardCacheImpl::UsingForegroundBackgroundCacheSizeLimit());
+}
+
+// Covers BackForwardCache's default cache size-related values.
+// Note that these tests don't cover the values configured from Finch.
+class BackForwardCacheDefaultSizeTest : public ::testing::Test {};
+
+TEST_F(BackForwardCacheDefaultSizeTest, DefaultCacheSize) {
+  EXPECT_EQ(BackForwardCacheImpl::GetCacheSize(), 1u);
+  EXPECT_EQ(BackForwardCacheImpl::GetForegroundedEntriesCacheSize(), 0u);
+  EXPECT_FALSE(BackForwardCacheImpl::UsingForegroundBackgroundCacheSizeLimit());
+}
+
+}  // namespace content