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?

Standover height doesn't mean squat. What really matters is cockpit length (ie - top tube length + seat tube angle = reach). I'm 5'9" and ride a 19" X-Cal 29er. Top tube is a little high, but the rest of the bike fits much better.

It probably really comes down to: do you want the bike to fit well or do you want to have a slight risk of racking yourself? Test ride different sizes to see what feels right to you...and get clipless pedals.

1+ to Yotsko, you need to ride and try for yourself. I am 5'9 but the way I am built, My X-Cal 29r is a 17.5. I can ride either one but the 17.5 with a tad longer stem worked better for me.

john - the x-call is the *****, isn't it? Just curious, did you flip your stem?

I loved it, but I should have said WAS..... the frame cracked and that turned into a nightmare all its own, so I moved all the stuff off it onto a Niner EMD frame. I still have a Paragon 29r which I find the geometry is very similar on. The point being my Paragon is also a 17.5. I did not flip the stem, but I put a longer stem with 0 deg rise ( on both ) and like them very well.

Exactly, your body could be proportioned like a tyrannosaurus or an orangutan. STandover is important though, if your sack is touching the tube on the flat you're gonna nail it the first steep up or down hill you have to dismount on.