I've always heard that the sump should be 1/3 the tank volume... although these "rules of thumb" are usually very vague...
A 55 gal should be "big enough" to hold plenty of media as well as the additional water when you turn the pump off... but as most things with aquariums... the bigger the better...
HI NEWBIE.....I would have to say 75-100g sump....Even on a big tank, I still rely on HOB(Hang On Back) filters to maintain the mechanical filtration.....Two AC101's will do a good job of circulation and mechanical filtration..