More specifically, the length from the center of the crank to the top of the seat tube.

This doesn't equate directly to a dimension on the rider. It's more like knowing your shirt size. And note that mountain bikes, for example, are normally used in smaller frame sizes than road bikes, so you don't just have one number that tells you what bike to use in all cases. A lot of cruiser-style bikes are normally sold and used in smaller frame sizes than road bikes.