TWO measurements are critical to start -- standover height (does your 'jewelry' touch the top tube when you straddle the bike?), and saddle height when properly adjusted (I use the heel-on-pedal method), making sure it doesn't take too much seatpost.
Pretty much all factory bikes have published stats and geometry on their websites. When you test-ride bikes, these are the first things you check. When you find one that feels right (or really CLOSE), get make/model/size, and go "Googling". Match those numbers as close as you can to the bike you're looking at.
I've ordered my frames online and built them up myself for a decade now; I know without measuring by now what geometry I need.
As long as you're going to do business with them, your LBS should help and support your research.