You go with a smaller diameter brake power booster only to clear tall valve covers with a big block in many smaller cars (the Corvette, Camaro and the Nova usually) that have marginal room in the first place for a big block. A big Chevy was one of the few models to get the full size big block (the Corvette that shared parts with and was split off from the full size Chevy originally to make it into a sportier model was the other) all others had to live with the 400 cubic inch limitation placed on them by the executive GM board.
To stuff a big block in a Corvette required a special valve cover with a dent on the drivers side valve cover to clear the eleven inch brake booster. All reproduction chromed stock looking valve covers for the BBC have this dent in them as being a Corvette part originally makes your car go faster (these chromed copies of the original valve covers are also all missing the oil drippers that were spot welded under the valve covers on factory original parts to oil the ball on the rocker arm).
Many think that a smaller chromed power assist booster must be better because they saw one on a Camaro with a big block that had tall valve covers. Idiots also install tall valve covers on their motors because race engines have used tall valve covers to clear the rocker arm stud girdles bolted to the top of their rocker arm studs that used to be in vogue with racers, before they all switched over to shaft mounted rocker arms with custom fabricated valve covers to clear the now wider instead of taller rocker arms. Copying a race engine (appearance, or the sound it makes with it's rough idle) is silly in my opinion for a car driven on the street. It can only hurt your street driven performance (called streetability) and possibly hurt you and others if you installed the wrong brake part just to look fashionable.
Bigger is better with brake boosters. That is to say you want the most available diaphragm surface area you can obtain. In passenger cars that is a eleven inch booster. In the aftermarket a DUAL nine inch brake booster with twin diaphragms instead of just one is better or bigger than the single eleven inch.