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I am going to buy a cycle. In this regard I got my framesize 20". Now I would like to know is it for 26er or 29er ?? Is frame size will be different for 29er and 26er ??
I am going to buy a GT Timberline. My height is 5' 11.5" inseam is 34.5". I am from Bangladesh. May be I am going to have 18.5" frame size.
When 29ers first became popular many buyers found that many manufacturers did not adjust their size charts to allow for the increased standover height. Whether this is the case for the models you are looking at is impossible to tell without more information. With identical main frame geometry, a 29er will be taller than its' 26" counterpart. You really have to test ride or test fit each to determine which size is correct for you. If you are comparing sizes between different brands there may also be a size differences. Brand "A" may require a size 20", Brand "B" may require a different size.
The frame size not only varies with wheel size, but also with manufacturer and type of bike. So, you need to know the frame size you need for the particular model of bike you're going to ride.
So, if you ride a 20" 29er with brand A, theres a non-trivial chance that brand B's 29er will be too big or small for you. And theres a good chance that if you ride a 20" 29er, a 20" 26er will be too small for you. If you have a regular old road bike which is a 58 cm, for example, you may find a 58 cm cyclocross bike too big for you. Examples go on and on.
The only way you can be sure of if a bike will fit you is to try it. The second best thing to do is look at the geometry chart (such as this one for the Trek Madone):
and see if the measurements are appropriate for you, for a given size. The two most important ones are typically (effective) top tube length and standover height.
There's no telling. You must get the dimensions of both bikes and compare. The 20" measure only tells you the (theoretical) seat tube length, and there are many other variables.
[On re-reading your question, it sounds like you've been told a 20" bike "fits" you. This is an approximate measure that primarily takes into account your leg length. It would only be a starting point when you walk into a bike shop and ask to try out a bike -- you need to actually ride the bike to see if it fits you in all regards.]
The frame size will not be different, as it's 20" on both. Only wheels will be different size.
