Bike: 2011 Specialized Hardrock Sport Disc 29er
My Height: Between 5'7" and 5'8"
My Inseam: 29.5"

I stood over a 15.5 and 17.5 frame on this bike in the store. My crotch was rubbing against the top tube of the 17.5 while I stood flat foot in a pair of Nike cross-training sneakers. On the 15.5", I had more clearance. Didn't check any other fitments beyond this. Guy at the shop said he'd go with the 15.5. In looking for posts from other people who have purchased 29er's and have very close my height/inseam, it seems most people ended up with 17-19, so I'm confused.

I've got to special order the color I want, but I can come back within 7 days and do a full refund exchange. I haven't ridden a bike in over 10 years, so I don't have any reference (certainly not for 29er sizing) to know how to pick the best size. Is 15.5 too small for me? Would the 15.5 or 17.5 be better?