I'm running a BBC with (I think) a 11" booster. I measured the entire over all diameter.
My master cylinder is leaking and I want to change it. I know NOTHING about how to do that so I'm hiring it done.

Due to not having much room in the engine compartment, I thought maybe I'd run a smaller booster.

I have a very radical cam so with this 11", I run a vacume canister to 'accumulate' vacume.

Can I get away with a 9" booster?
7"?

Not interested in running a vacume pump.

Seeking advice.

you could use a dual diaphragm 9" and it would give about the same amount of assist as the 11" (maybe even a little more?)

i read that an 8" dual diaphragm has about the same surface area as a single 11"

Thank you sir....

tiz the way I will go...

yup sure thing
i got a 8" dual for my car, it only had about 7" of vacuum in gear at idle
and with the new booster it works great
i searched here and asked and could not get an answer so i just tried it and have been very pleased

here is where i ordered it
http://www.hotrodsusa.com/
here is the booster i got
http://www.hotrodsusa.com/store/product328.html
and you will need the brackets near the bottom of the page
67-72 CHEV POWER BRAKE BRACKET:
http://www.hotrodsusa.com/store/boosterchev.html

This is highly encouraging. It will give me more room my current 11" booster is about an inch from the valve cover.