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?).
It depends on the manufacturer so you'll need to check the frame geometry specs. Even frames sized in inches or centimeters are not all the same, since different manufacturers measure between different points to describe frame size. There's usually a diagram on the frame spec page defining the different measurements.