What David just said, and he beat me to the punch posting that link. However be aware that there are differing opinions
about the finer points of frog systems and optional variations. There's at least a hundred topics on the forum about the finer points of frog systems!
Quick and dirty method for footloop length - with loop(s) already tied, measure floor to belly button plus a little extra to allow for knots.
Better method, get your gear on and hang on rope. At full extension of the legs, the cam on your upper ascender should about touch the cam on your chest ascender attached to your seat harness.