@TestOfOath the 6 inch would suffice if it has been for 3 years. Seems like that’s a decent amount of time with a exhaust fan. Especially if it’s always running.
We do have the bundles on the site of the fan and filter combo, the exhaust fans are usually setup around the duct port sizes on a tent or light, as to not have to use reducers, which will speed up failures.
So if the port sizes make sense to go 8" then that would allow for a greater amount of area being moved.
The 4" is imo maybe good up to a 3x3 space. Maybe 4x4 if you got the right room around it, but as the size goes up the amount of air being moved is reduced greatly. Inline booster can also solve that issue if the fan isn’t enough for a given space.