I have a 68 Chevelle... I have converted (almost) from 4 wheel drum to Disc/Drum, does the size of the booster matter? I see 7" booster, 9" boosters... will they all work?

The stock booster is actually 11", but a 9" will work just fine provided the master cylinder will bolt up correctly. Also don't forget to install the proper rod in the booster that goes into the master cylinder.