How do I figure out if this bicycle would be a decent height for a person that is 5 feet 8 inches in height?
Next to the M and L, it lists 65"-70", and 68.5"-75". " is an abbreviation for inches. So at 5'8" (5 feet 8 inches), or 68 inches, the M is probably a better fit, but if the rider is still growing, you might get the large because it's so close.
That said, it seems to be a very expensive bike to be buying without a test ride first, even more so when you are on the border between sizes. I'd suggest looking at something locally if that's an option, or at least something with plenty of good reviews if you can't buy locally.