Factory brake booster question

fireguyfire said
Where do I measure to determine the size of booster that I have?
Mine is the original and my car is a formulas S 67 with a commando engine.

View attachment 1716304247

It's the diameter of the housing, give or take a bit.
A couple of pictures from Harmon's website for clarification:
6 1/4:

7 1/2: