63 is about as large as you find off the shelf now, which is about equal to the 25.5" bikes of old when the "size" was the length of the seatpost tube instead vertically to the top bar from the bottom bracket. Maybe avoid a 63 with a down-sloping tube if you don't like a lot of exposed seatpost. For a vintage bike, Panasonic actually made a 27" bike years ago--that probably would be about 68c now. I had one around about the '79-ish time period; however, I have not had a problem with 25.5" and 63c bikes since then, and even "L" instead of "XL" MTBs.