i have another question - without Washington treaty,how much and how big superbattleships would be made ?
I personally would love to see german superbattleship with Dora/800mm cannon/ as main battery.
The problem is that battleship cannons in excess of 16"-18" caliber are incredibly inefficient. The ballistic range advantage of larger calibers ceases to be relevant because WWII-era fire control cannot see and aim any further, and the rate of fire declines exponentially with shell caliber -- whereas 16" guns fired only a little more slowly than 15" guns (2-2.5 rounds per minute versus 2.5-3 rounds per minute), 18" guns were substantially slower at 1.5-2 rounds per minute, and 20" guns were *optimistically* estimated at maybe one round per minute.
The largest battleship ever designed even on paper was the German H44 concept, which would have displaced 140,000 long tons at full load and been armed with eight 20" guns in four twin mounts; however, this was a purely theoretical design which was literally too big to actually be built in any shipyard in Germany, and also too big to enter any harbor in Germany.
The largest battleship to actually be approved for construction was the Montana class, at 71,000 long tons full load with twelve 16" guns in four three-gun mounts. A hypothetical no-limits super battleship could be bigger than Montana, but certainly not as big as H44. Perhaps a hundred thousand tons with fifteen 16" or twelve 18".