diff --git a/git-p4.py b/git-p4.py
index 37265fc31d..dc00946f46 100755
--- a/git-p4.py
+++ b/git-p4.py
@@ -47,8 +47,8 @@ def __str__(self):
 # Only labels/tags matching this will be imported/exported
 defaultLabelRegexp = r'[a-zA-Z0-9_\-.]+$'
 
-# Grab changes in blocks of this many revisions, unless otherwise requested
-defaultBlockSize = 512
+# The block size is reduced automatically if required
+defaultBlockSize = 1<<20
 
 p4_access_checked = False
 
@@ -958,7 +958,8 @@ def p4ChangesForPaths(depotPaths, changeRange, requestedBlockSize):
     changes = set()
 
     # Retrieve changes a block at a time, to prevent running
-    # into a MaxResults/MaxScanRows error from the server.
+    # into a MaxResults/MaxScanRows error from the server. If
+    # we _do_ hit one of those errors, turn down the block size
 
     while True:
         cmd = ['changes']
@@ -972,10 +973,24 @@ def p4ChangesForPaths(depotPaths, changeRange, requestedBlockSize):
         for p in depotPaths:
             cmd += ["%s...@%s" % (p, revisionRange)]
 
+        # fetch the changes
+        try:
+            result = p4CmdList(cmd, errors_as_exceptions=True)
+        except P4RequestSizeException as e:
+            if not block_size:
+                block_size = e.limit
+            elif block_size > e.limit:
+                block_size = e.limit
+            else:
+                block_size = max(2, block_size // 2)
+
+            if verbose: print("block size error, retrying with block size {0}".format(block_size))
+            continue
+        except P4Exception as e:
+            die('Error retrieving changes description ({0})'.format(e.p4ExitCode))
+
         # Insert changes in chronological order
-        for entry in reversed(p4CmdList(cmd)):
-            if entry.has_key('p4ExitCode'):
-                die('Error retrieving changes descriptions ({})'.format(entry['p4ExitCode']))
+        for entry in reversed(result):
             if not entry.has_key('change'):
                 continue
             changes.add(int(entry['change']))
diff --git a/t/t9818-git-p4-block.sh b/t/t9818-git-p4-block.sh
index 8840a183ac..ce7cb22ad3 100755
--- a/t/t9818-git-p4-block.sh
+++ b/t/t9818-git-p4-block.sh
@@ -129,6 +129,7 @@ test_expect_success 'Create a repo with multiple depot paths' '
 '
 
 test_expect_success 'Clone repo with multiple depot paths' '
+	test_when_finished cleanup_git &&
 	(
 		cd "$git" &&
 		git p4 clone --changes-block-size=4 //depot/pathA@all //depot/pathB@all \
@@ -138,6 +139,13 @@ test_expect_success 'Clone repo with multiple depot paths' '
 	)
 '
 
+test_expect_success 'Clone repo with self-sizing block size' '
+	test_when_finished cleanup_git &&
+	git p4 clone --changes-block-size=1000000 //depot@all --destination="$git" &&
+	git -C "$git" log --oneline >log &&
+	test_line_count \> 10 log
+'
+
 test_expect_success 'kill p4d' '
 	kill_p4d
 '