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I am 5'8" and 145 lbs. I recently bought a specialized rockhopper 15" as I was told that this is was the best fit. I am not real sure how i am supposed to feel but is the 15" to small for me? please help so i can return if possible. thanks

The store should aid you in proper fitting as part of the sale process. MTB fitting is different from most, so perhaps try the MTB forum for best advice.

https://www.wrenchscience.com has a complete sizing procedure to find out if the bike is right for you. Lots of measurements, some of 'em wierd, but it should get you where you want.

PS It might be for road only.

I'm 5'5". Trek Comfort bike, 16.5 inches. Kona Caldera hardtail, 16 inches. Gary Fisher Cake full suspension, 17.5 inches. Depends upon the frame design, your leg length, your riding style. They all ride differently, and I use them differently. For me, these bikes are fine.

FYI 15 inches is 38.1 centimeters. The bike "sounds" small to me, but what the heck do I know.

You should have 3-6 inches of clearance between your crotch and the top tube when standing on the ground over your mountain bike. I would think a 17 might be a better fit depending on your inseam.

.....but it's a Mtb frame so it sounds "about right"...