The Yamato was 862 ft long, 121 ft wide, 68,010 tons standard displacement, and had 9 18.1 " guns.
The Iowa was 887 ft long, 108 ft wide, 52,000 tons standard displacement, and had 9 16" guns.
The Bismark was 823 ft long, 118 ft wide , 45,200 tons standard displacement, and had 8 15" guns.
As you can see, the Bismark wasn't the biggest battleship of WW2, but, she was notorious. Her sinking was famous. It should be noted, that althought the Iowa's had smaller main guns than the Yamato, the 16" guns actually had more armor penetration than the 18.1" guns.
Here's a good site the compares battleships of WW2. It covers armor, guns, radar, AAA, and more.
http://www.combinedfleet.com/baddest.htm