Hygrorhyza aristata is a nice one.
I've got "dwarf" water lettuce as well and it's probably 8" across.
The dwarf water lettuce is a strange one isn't it. In my high tech tank with EI ferts and fairly high light, it stays small and throws out loads of daughter plants. In my Moina tank that has only residual traces of ferts and fairly low light, it gets massive!