What is the measurement in inches of a XL frame? What is the conversion of a (XL) frame to it's measurement in inches?
It depends on the particular frame - XL doesn't mean a specific size. Also, even for frames labeled as the same size number (say 23" or something), thats only one measurement to summarize the size of the bike - there are many more which can make a bike fit or not - top tube length, stand over height, etc. 



(Look at all of the quantities labeled on the figure, courtesy of Trek).

The two most important though are top tube length (E) and standover height (B?).