Well, I am just over 5'9" tall and a 54cm fits me perfectly when riding the bike (slight layed back seatpost and 100mm stem). It is true that the standover height is greater than what a normal 54cm road frame would be. When I am standing over the top tube, it's really snug - even taller than my 56cm conversion.

I'm no expert and I don't have a 56 to measure against but I'm guessing a 54 will feel really small to you. Try to find a shop that has one built up so you can check it out in person.

Good luck.