If you download the RT brochure
here and scroll down to page 9, you'll find frame dimensions and some advice about sizing. It depends on your riding style, though, and whether you want the current straight bars or drop bars.
I bought a s/h RT which like the above was a Short frame (generally intended for drop bars) with straight bars on it. I found it far too short and upright. It wasn't a problem though as I prefer drop bars anyway and once I'd made the change over I found it very comfortable. (I fitted Origin 8 bar-ends which work fine.)