There are lots of old road bikes laying around that can be had for pocket-change. Buy one in the size you want to try and fiddle with it. I did this a year ago and found that I have probably been riding a too-big frame most of my life. I am a hair under 6'3". I can do most all riding with a 25" frame very well, but I have found that an inch smaller frame has benefits in fast riding positions and comfort.