The standover height on bikes can vary - but the MAIN size thing is (ignore my vocabulary here) the distance from the seat to the handlebars - reach? Put the two together (standover & reach) to arrive at a comfy fit. If you're reaching too far (as you had indicated in a different post) your back is goning to hurt. I would honestly suggest you visit a local bike shop (not REI, which is more a sporting goods store) and check out the different "fits" of different sized bikes. Tell them you're shopping. After you do this, you may have a better idea about trading your 17" in at REI for the 15" or not.
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