Measure your inside venturi bore area of the carb. That would tell you what your stock carb is in relation to one of the Perry carbs
But yes, a smaller bore improves fuel draw and a larger bore reduces fuel draw, abeit a smaller bore might reduce top end power some and a larger bore might improve the top end some.
So you measure the bottom outside base of the carb to match your engine's carb hole and then the inside bore of the carb is next to determine which one you want to get.