U
User6179
Guest
Except the effective TT is 59cm
They measure by seat tube looking at the grid (as I think do Boardman/Hellfrauds)
A medium would make sense for 5'9, ~55cm TT (54.7 cm for the pedant)
Rider Height Suggested Frame Size
5′3″ to 5′7″ (160cm-170cm) Small
5′8″ to 5′11″ (173cm-180cm) Medium
5′10″ to 6′2″ (178cm- 188cm) Large
6′1″ to 6′6″ (185cm- 198cm) X-Large
I think buying blind could be pot luck depending on lay-back of seat post and the stem size and if the bars are straight or sweep back and your leg length.