I usually get boards in the smaller sizes. Its a lot easier to make the hole bigger, than to make it smaller. Also, there were 2 or perhaps 3 different "Copal 3" sized shutters. I have a 60mm and 63mm and there is also a 65mm shutter that all say "Copal 3."
The "Horseman/Sinar" board will, of course, take a #3 sized hole, but the 8x8cm Horseman boards won't.