I have seen a lot of questions asked about frame sizing, in this forum and elsewhere. Most experienced cyclists know the distance between the center of bottom bracket, to the the top of the saddle, measured in line with the seat tube, off by heart.
For anyone who is purchasing his first road bike, I caution that perfect frame sizing is not something you can do over the telephone, or by e-mail. You can also get it wrong by using the fitment programmes available on computer. You have to go to an experienced professional cycling shop and get measured and fitted properly. This will also take care of the correct handlebar stem, and bar. Even different saddles and seatposts can affect your position on the bike. Some shops have a jig, which is set up as a bike with various adjustments. You simply get onto the jig and adjustments are made until you arrive at the correct fit. From there you can best determine your preferred geometry and size.
One fellow here, has suggested sizing your frame, by measuring the top tube. While this may work sometimes, if you have an existing bike set up perfectly and you want to match this, it still presents some margin for error. If you use top tube measurements for your first frame, you are likely to get it wrong.