7" 90 - 200 CFM
10" 200 - 450 CFM
i'd say go with 10'' as 200cfm is to close to what the 2.5 requires.
there are some spinner ones as well that apparently are the best for spitting out the dust rather than catching it, they are expensive though. the plastic ones get sandblasted inside, not sure if there is a glass version of those
this is all from memory from when i was looking at a cap for my snorkel - i was also considering a sock (the knn type) that goes over the filter but have it over the snorkel cap instead - not sure how that would work though.