These are a fast growing fish, given good tank conditions and feed regime...They can grow up to anywhere between 4 - 6 inches in captivity...55, or even 75 in my opinion...
The main thing with these is not overall swimming room, its about dillution...They are VERY messy, and the larger tank size is desirable because of this, as they are not that an active swimmer...
At 2" now, you could keep that angler in a 20 gal long for awhile. Just keep up with water changes and maybe use a skimmer. Like stated, they aren't active swimmers. They also shouldn't be feed every day. Three times a week should be ok.