6 inch
Seems that my 6 incher was the one that was always in my hand. I am a machinist, so I've got loads of them. You might consider a 3 piece set from one of the import suppliers. They are surprisingly accurate, and of good quality. If you are buying a machinist combo square,( with sliding head) I would invest in a 12 inch Starrett. The imports I have seen are far inferior.
Regards,
Chris