A subwoofer is 10" or larger. (Yes, I know there are some high-end 8"

Is there like a standard that says that? I have a reasonably good quality 8" speaker (subwoofer?) in a sealed enclosure that rounds out the bass tones nicely. I can't tell much difference between it and my friends that have a 10" subwoofer, but admittedly, I haven't done any objective testing.