There are any number of "fit calculators" on the web that will get you within the ball-park concerning saddle height, reach to the handlebars, etc. I wouldn't take any of the fit calculators as the Gospel Truth, but they'll get you pretty close (+/- one centimeter, I'd imagine).

If you can get your bike setup to be what the fit calculators say, and you have a bit of "wiggle room" either way to adjust for your preferences, I wouldn't worry about the frame size or how much seatpost is showing, etc.