From what I can tell a 50 vs 53cm is a matter of preference, not correct sizing. At 5'8" with a 31" inseam I have most of my height in my legs. If the Kilo has a longer tt I'd size down. Judging by the geometry specs on the Bike Island site, the standover of the 50cm is 772cm and on the 53cm its 800cm. 31 inches = 787 cm. So yeah.