Bike Frame Size versus Bike Fit

No, ideally, as I have been learning, you get "fit" first.  IOW, get a rec for a local bike fitter (FIST certified), go there, they will set you up on a "bike", take all your measurements, angles, etc.  Then you should know what type of geometry you need, and THEN find a bike that can satisfy your geometry.  Yeah, I though it was the other way around as well.

Think of size as the "macro" adjustment and fit as the "micro." If you were of an older generation, I would compare it to the old rotary tuner televisions. You would click the dial on the right channel, say Channel 6, which would get you into the neighborhood, then use the fine-tuning control to get the best reception. But, no matter how much fine tuning you did, if you were on Channel 6 you could not get Channel 7 or Channel 5.

On a bike, say your appropriate size is a 54cm. From that point, a good fitter can manipulate seat height, stem length and height, crank length, handle bar width and angle, and other variables to get you into the most efficient and comfortable position. However, if you need a 54 and start with a 56, and start making major adjustments, it could be detrimental to bike handling and create other issues.

The risk, then, is that you get the wrong size. Best approach is to try several sizes of the same model, all set in a comparable adjustment, to see what feels best. Of course this is easier said than done, and further complicated in that different manufacturers size their bikes differently, there is very little industry standardization. Probably your best insurance is to start with an LBS that has a solid reputation for sizing and fit, so that they know how one effects the other.