diff --git a/docs/backup.md b/docs/backup.md
index 6142e5b..5b796f7 100644
--- a/docs/backup.md
+++ b/docs/backup.md
@@ -281,12 +281,13 @@ recipients as normal backup shards, and are committed with messages like
 to use the root `manifest.json` as the authoritative completed backup. This
 keeps long runs crash-tolerant without pretending partial data is a finished
 snapshot. A checkpoint commit can cover many messages, but its encrypted files
-are split into smaller shard files to stay below normal GitHub blob limits. Tune
-the commit cadence with `--gmail-checkpoint-rows` / `--gmail-checkpoint-interval`
-on `gog backup push`, or `--checkpoint-rows` / `--checkpoint-interval` on
-`gog backup gmail push`; set the interval or rows to `0` to disable that
-trigger, or use `--no-gmail-checkpoints` / `--no-checkpoints` to disable
-checkpoint pushes entirely.
+are split by both row count and a conservative plaintext byte ceiling so large
+messages do not create GitHub-rejected blobs. Tune the commit cadence with
+`--gmail-checkpoint-rows` / `--gmail-checkpoint-interval` on `gog backup push`,
+or `--checkpoint-rows` / `--checkpoint-interval` on `gog backup gmail push`;
+set the interval or rows to `0` to disable that trigger, or use
+`--no-gmail-checkpoints` / `--no-checkpoints` to disable checkpoint pushes
+entirely.
 
 `--include-spam-trash` defaults to true. Use `--query` and `--max` for bounded
 test exports; omit them for a full mailbox scan.
diff --git a/internal/cmd/backup_gmail.go b/internal/cmd/backup_gmail.go
index c314487..b6569ca 100644
--- a/internal/cmd/backup_gmail.go
+++ b/internal/cmd/backup_gmail.go
@@ -353,6 +353,8 @@ type gmailBackupCheckpointer struct {
 
 const gmailCheckpointShardMaxRows = 250
 
+var gmailCheckpointShardMaxPlaintextBytes int64 = 32 * 1024 * 1024
+
 func newGmailBackupCheckpointer(ctx context.Context, opts gmailBackupOptions, total int) *gmailBackupCheckpointer {
 	enabled := opts.Checkpoints &&
 		opts.CacheMessages &&
@@ -876,21 +878,57 @@ func buildGmailCheckpointShardsFromCache(accountHash, runID string, firstPart in
 		return nil, nil
 	}
 	shards := make([]backup.PlainShard, 0, (len(ids)+gmailCheckpointShardMaxRows-1)/gmailCheckpointShardMaxRows)
-	for start := 0; start < len(ids); start += gmailCheckpointShardMaxRows {
-		end := start + gmailCheckpointShardMaxRows
-		if end > len(ids) {
-			end = len(ids)
-		}
-		shard, err := buildGmailCheckpointShardFromCache(accountHash, runID, firstPart+len(shards), ids[start:end])
-		if err != nil {
-			for _, shard := range shards {
-				if strings.TrimSpace(shard.PlaintextPath) != "" {
-					_ = os.Remove(shard.PlaintextPath)
-				}
+	chunk := make([]string, 0, gmailCheckpointShardMaxRows)
+	var chunkBytes int64
+	cleanup := func() {
+		for _, shard := range shards {
+			if strings.TrimSpace(shard.PlaintextPath) != "" {
+				_ = os.Remove(shard.PlaintextPath)
 			}
-			return nil, err
+		}
+	}
+	flush := func() error {
+		if len(chunk) == 0 {
+			return nil
+		}
+		shard, err := buildGmailCheckpointShardFromCache(accountHash, runID, firstPart+len(shards), chunk)
+		if err != nil {
+			cleanup()
+			return err
 		}
 		shards = append(shards, shard)
+		chunk = chunk[:0]
+		chunkBytes = 0
+		return nil
+	}
+	for _, id := range ids {
+		msg, ok, err := readGmailBackupMessageCache(accountHash, id)
+		if err != nil {
+			cleanup()
+			return nil, err
+		}
+		if !ok {
+			cleanup()
+			return nil, fmt.Errorf("gmail message %s missing from backup cache", id)
+		}
+		line, err := json.Marshal(msg)
+		if err != nil {
+			cleanup()
+			return nil, fmt.Errorf("encode gmail backup checkpoint shard estimate: %w", err)
+		}
+		lineBytes := int64(len(line) + 1)
+		overRows := len(chunk) >= gmailCheckpointShardMaxRows
+		overBytes := gmailCheckpointShardMaxPlaintextBytes > 0 && len(chunk) > 0 && chunkBytes+lineBytes > gmailCheckpointShardMaxPlaintextBytes
+		if overRows || overBytes {
+			if err := flush(); err != nil {
+				return nil, err
+			}
+		}
+		chunk = append(chunk, id)
+		chunkBytes += lineBytes
+	}
+	if err := flush(); err != nil {
+		return nil, err
 	}
 	return shards, nil
 }
diff --git a/internal/cmd/backup_test.go b/internal/cmd/backup_test.go
index 420c45b..f8b3b66 100644
--- a/internal/cmd/backup_test.go
+++ b/internal/cmd/backup_test.go
@@ -454,6 +454,36 @@ func TestBuildGmailCheckpointShardsFromCacheSplitsLargeChunks(t *testing.T) {
 	}
 }
 
+func TestBuildGmailCheckpointShardsFromCacheSplitsByPlaintextSize(t *testing.T) {
+	t.Setenv("HOME", t.TempDir())
+	oldLimit := gmailCheckpointShardMaxPlaintextBytes
+	gmailCheckpointShardMaxPlaintextBytes = 1
+	t.Cleanup(func() { gmailCheckpointShardMaxPlaintextBytes = oldLimit })
+	accountHash := "accthash"
+	ids := []string{"m1", "m2", "m3"}
+	for _, id := range ids {
+		if err := writeGmailBackupMessageCache(accountHash, gmailBackupMessage{ID: id, Raw: strings.Repeat("raw-"+id, 8)}); err != nil {
+			t.Fatalf("writeGmailBackupMessageCache: %v", err)
+		}
+	}
+	shards, err := buildGmailCheckpointShardsFromCache(accountHash, "run-test", 11, ids)
+	if err != nil {
+		t.Fatalf("buildGmailCheckpointShardsFromCache: %v", err)
+	}
+	if len(shards) != 3 {
+		t.Fatalf("len(shards) = %d, want 3", len(shards))
+	}
+	for i, shard := range shards {
+		if shard.Rows != 1 {
+			t.Fatalf("shards[%d].Rows = %d, want 1", i, shard.Rows)
+		}
+		want := fmt.Sprintf("part-%06d.jsonl.gz.age", 11+i)
+		if !strings.HasSuffix(shard.Path, want) {
+			t.Fatalf("shards[%d].Path = %q, want suffix %q", i, shard.Path, want)
+		}
+	}
+}
+
 func TestBuildGmailMessageShardsFromCacheWritesPlaintextPaths(t *testing.T) {
 	t.Setenv("HOME", t.TempDir())
 	accountHash := "accthash"