it's important to know what you're measuring, whether it be the seatpost (center to center) or something else. i'm almost 5'11" with a 32" inseam and ride 22" (56cm) bikes measured by the seat tube c-c. this provides 4" of seatpost exposed, which looks rather perfect. for those 5'8", look for a 52cm bike measured c-c or maybe something slightly longer when measured c-top.