All depends upon your inside leg, and of course there is a huge difference between the older design horizontal top tubed bikes and the newer compact frames.
A 21 inch would be OK if a horizontal top tube (say 31/32in inside leg), but not if a compact it would be way too big - go to a bike shop or bigger retailer and mooch about and ask them what is probably best.....say you'll have a think, nothings taken your eye, and get some more advice. Try some for size. As a guide, the seat post should be way out of the frame, not with nothing showing, but well within the limit marker !