Forget about the term "frame size" entirely. If you're looking at brands that list the stack and reach, that's all you need to define the fit. Rather than inseam, which is often mismeasured, go by your actual saddle height, assuming that you have a bike with a properly adjusted saddle height.
As an example, if your cycling inseam (not pants inseam) is 34 inches or 86cm, then your saddle height should be around 76cm. From there, a saddle to bar drop range, and an appropriate stack height can be figured. A stack height of 555-560mm would provide a racing fit with a saddle to bar drop in the 7-11cm range, depending on the stem angle. If a more recreational fit is desired, then increase the stack height by 20-30mm.
If a brand does not list the stack and reach, then you can only compare head tube length and TT length, with corrections needed for any difference in the seat tube angle. Steeper STAs make the reach longer, by about 1cm per degree.
On pitfall to the reach concept is that it is only valid at ONE stack height. If you're comparing frames with different stack heights, then corrections must be made to the reach. A taller stack height reduces reach by about 3mm for each 10mm of additional stack height.
http://www.cervelo.com/en_us/bikes/2012/R3/geometry/