Plenty of head room to to 12-14 gallon batches. Still small enough for a typical 6 gallon batch. ( I think)
The mash tun is going to be an issue trying to do a 6 gal small beer in a 20 gal pot. Likely the bed will be too shallow ( < 4" ) and the volume won't be large enough to reach the thermometer stem (or sensor) so you won't know the temp without using a manual immersion thermometer.
You can overcome that, but be aware of it.
I have (ahem) 3 mash tuns: A 20 qt Tremonti (for 5 gal beer up to 1.060), a 10 gal Blichmann (for 10 gal small beer or 5 gal big beer) and a 1/2 barrel keggle (for 10 gal big beer or 5 gal barleywine). I don't see any way to get around it if I want both small and big beer in 5 and 10 gal batches.
Charlie (doesn't brew a 10 gal big beer unless he really likes it)