I think I've pretty much narrowed it down to 58cm and 60cm, and I'll see which one feels best for me. However, couldn't I replace the stem with a shorter stem on the 60cm and then if I grow another inch or two, put the original stem back on?

Also, like I said, I'm 5 foot 11 1/4 inches, but my arm span is 6 foot 2 1/4 inches. Would this play a factor? My mountain bike is 19.5 inches and it fits me very well. It's a Trek Fuel EX 5. The road bike I'm looking at is the Trek Emonda SL 5.