Thanks for asking Mike, this is helpful to everyone...
Measure around your wrist in inches. If you don't have a flexible ruler, use a piece of string and lay that against a straight ruler. This size guide was found online and I adjusted it because I think stock sizes from online retailers are ridiculous. Panerai straps should have length and thickness IMO. With a 7" wrist, a 125x75mm strap that is around 4-5mm thick, will have the tail fit through one thick keeper and that's about it. Too small for me. This is a rough estimate for my straps.
If your wrist is..........
6.00 to 6.5in. - 120/75mm
6 ½ to 7in. - 125/80mm
7.0 to 7.5in. - 130/85mm
7.5 to 8.0in. - 135/90mm
8.0in. + - 140/85mm - 140/90mm
MM means millimeter and there is approximately 24mm in 1 inch. Most watch measurements are in millimeters because it is a lot easier to use this, rather than saying 1/24th of an inch. Lug distances are measured this way for watch strap sizing. Standard sizes range from 18, 20, 22, 24, 26 etc...
I have a 7" wrist and like my straps around 130x85mm. This leaves the shorter side in the center of the bottom of my wrist and a tail like this:
Let me know your wrist size and I will let you know what I think. If you have a strap you wear now and like it, just measure that.