actually, for one thing, when a bike's size is being described, such as 20", 24", or 26", it is describing the wheel diameter, not the frame size itself. a 17" bike is too small for an adult. an adult bmx bike is 20", so a 17" bike would be smaller than that. a boy's bike is 16". you can probably fit a 26" bike if you put the seat all the way down. go to walmart, pull a mens 26" bike off the rack, put the seat down, and try to sit on it. its easy to do, look under the seat for a quick release lever, release it, and push the seat down. you may have to wiggle the seat to get it to go down. as long as you can reach the pedals, youre good. also, unless you are buying a shift kit, which i wouldnt recommend for a beginner, it doesnt matter if its a mountain bike or a beach cruiser. the engine does not use the bikes gears. if using a shift kit, the engine does use the bikes gears, but shift kits are very expensive. it costs more than the engine kit does. i personally use beach cruisers, which are 26" single speeds, for all my bikes. my advice, as new as you are and as little as you know about bikes, i would keep this project as simple as possible.