I’m trying to get a mountain bike online for cheap, but don’t know what dimensions to look for since kids bikes look the same as mens bikes with out a frame of reference? I’m 6′ 1″ tall and about 200 lbs…please help. These people online have all told me I would fit comfortably on their bikes w ith the frames ranging from 21″ – 26″??? I reflect they are just trying to sell their bike to some sap 
SpareHand Triple Bike Parking StandThis Triple bike stand can be used either as parking stand for 1 bike, 2 bikes or 3 bikes. Durable chrome end will g… More >>
Polaris Edge LX20 Kids 20-Inch BikeWith the Edge LX 20 built on the “T” frame and equipped with a hand brake and a coaster brake, kids will keep safely in … More >>
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get thee to your local bike shop and “get fitted” to ensure the right size and set up for you
try to stay away from “cheap”, it will be better for you in the long run
check out KHS, Trek, Specialized, and Giant
The person above me took the words outta my mouth! Fit is CRITICAL!!! Get to the bike shop. Do it. NOW!!! Hehehee…
Delight in your new wheels!
As a retired bike dealer, I can tell you that fit is very vital. I would recommend getting fitted by you local dealer…but dredge up, all bikes are not the same size. There are really 2 measurements involved…1st is wheel size…that’s the 12,16,20,24,26,and 27 inches . Then there is frame size…ths distance from the top of the seat post of the frame to the middle of the crank. How the bike isdesigned can make a difference in this measurement. There is also stand-over height and leg length. You must be able to stand flat footed over the frame and bend your knees a small (If you fall or have to jump off in a rush you are going to bend your knees, so make sure you have enough clearence.) Most of the excellent manufactures will have these measurments readily available.
Don’t let anyone tell you that you can raise the seat or place in a longer seatpost. That will not raise the handlebar height, and you will bent over too much, and not comfortable.
Finally, are you going to ride off road, or just around town? The rougher you are going to ride, the smaller the frame you can handle (see above about falling off.)
Go see your local dealer.