Is a 700c wheel size okay for adults?

Wheel size hasn't got a thing to do with it. It's the frame size that matters. You had better be fairly tall to ride a bike with a 21" frame size. Six feet tall or more. Go physically look at the bike & by all means if possible, test ride it.

there are several different wheel sizes used for adult bikes. What is important is the frame size that you need. You do not state either your height and inseam or the actual make and model of the bike in question. A 21 inch frame would be quite large if it is a hybrid bike and somewhat smaller if it is a road bike. 165cm-190cm is a large range in height. No single bicycle can provide a good fit for someone over that entire range of heights. Sounds like a typical discount store bike, one size fits some but by no means all. If you want to know if the bike is suitable for you, provide better and more complete information

The 700c wheels are standard for road and hybrid bikes what you want is the proper frame size.

700c is the standard wheel size for adult bikes. Road bikes, anyway, not mountain bikes, they use a different wheel size.

It is the frame size that matters. The frame size is the length of the seat tube, and correlates to your inside leg measurement (it is not the SAME as your inside leg though)

http://www.evanscycles.com/help/bike-sizing

http://www.wiggle.co.uk/h/option/bikesizeguide

http://www.ebicycles.com/bicycle-tools/frame-sizer

Are you riding a hybrid or road bike . I'm 5'8 with a 20 inch frame hybrid bicycle and could possibly best fit a 56-58 inch road bike best so .......