Nominal frame size is measured along the seat tube from the center of the crank spindle to either the center of the top tube (c-c, great for specifying frame geometry/angles) or the top of the top tube (c-t, a more pratical guide to how the frame fits the rider, although one has to add bottom bracket elevation to obtain standover height). Top tube length (almost always measured c-c) is arguably even more important in determining how well the bike fits you.

For what it's worth, I am 5" shorter than you and my inseam is 2" shorter, so our respective relative proportions are comparable and pretty average. My ideal size in a traditional European road bike is 55cm C-T.