Grandslam bombs were 22,000lbs, but required a Lancaster Special to carry it. The normal Lancaster could carry the Tall Boy which was 11,000lbs and likewise would break the sound barrier on the way down (I've seen footage of what the Tall Boys could do to fortifications, and it ain't pretty).
I understand that HTC has stated that the largest single bomb that will be available in AH will be 4,000lbs. The standard Lancaster payload was made of these and additional smaller bombs.
Sisu
-Karnak