I asked this in a previous thread about cam size vs vacuum, but I figured that it would be more appropriate to start a new thread instead.

The question I have is does carb (venturi) size have anything to do with the amount of vacuum you'll have? The way I figure, the source of your vacuum is the piston moving downard on the intake stroke and the resulting vacuum would have to be a function of any restriction to air flowing in to occupy the now empty space. Smaller venturis are effectively the only restriction (when you're not decelerating - i.e. closed throttle blades) that can induce manifold vacuum, so wouldn't their size play a part in the amount of vacuum you have?
My application is a 10.5 to 1 489 with AFR heads, probably an HP 950 or 1000 and a manual trans. I'm hoping that the man trans and the fact that I will be switching to a dual diaphragm booster will allow me to run a cam that has 250 to 260 something duration at .050, probably between .660 and .700 lift on a 110 or 112 LSA with only a vacuum can. Any thoughts?

At part throttle, it's not the venturis that are the restriction, it's the throttle. At WFO, the venturis may be the restriction--could also be the air filter or air cleaner housing. As long as the fuel mix is good, "any" manifold vacuum at WFO max RPM is "bad" for power. I've heard about 3/4"--1" is about the maximum you'd accept.

The vacuum produced at part throttle has more to do with throttle position, camshaft, displacement, ignition timing, and fuel metering than venturi size; but venturi size is a factor in fuel metering--a large venturi will tend to have poor fuel metering at low flow. There are fixes, notably annular boosters and other carb wizardry.

In general carb venturi size does not affect vacuum with respect to brake operation. What you *will* see though is that as carb venturi size is increased the vacuum changes with throttle change become wider. A very small change in throttle position will result in a large vacuum change with large carb venturis. This can in turn make throttle regulation a bit dicey on the street and engine response a bit on/off.