not sure what type of disk lock youre looking at, but if you go with the 'stapler' style you wont need to worry about the size of the pin, and really the bigger the better. the reason i say this is you dont need the pin to go through one of the holes or slots in the rotor itself.

heres how i have my disk lock


so as long as the slot in the lock is long enough he pin size wont matter
and definitely get the reminder cable, dont want to try and take off with the lock still on.

 

I'm not a fan of disc locks - a friend of mine had one fitted to his bike and forgot it was there one day - bike fell over and disc was warped - not a cheap fix on a Ducati
If you do buy one, get the one with the bright yellow reminder "string" on it and always make it visible so you don't forget it's there.
My 2c worth..