i don't know what the 56cm in your description is in reference to, but looking at the length of that headtube, i would guess the effective seattube length to be about 50cm. but i'm sure that a number of pro's at 5' 11" could and do use a frame of that size, but most recreational riders at 5' 11" would buy a larger frame.

i say this because frames where the toptube and downtube meet, like the one shown, before they reach the headtube usually have and effective seattube length of about 50cm or less, IME.