Chiming in with others here, but for what you are doing, 15 gal seems to fit the bill. That will give you a scale from 5 gal extract to 5 gal BIAB to 10 gal BIAB (if your grain bill is light enough).
For reference, I have a 16 gal pot and I do mainly 5 gal BIAB (though I did one extract full...