Average size 12-14 inch? They stay smaller then that. Maximum size would be 12, maybe 13 inch.
My Green Terrors are in a 190 gallon, and I change a lot of water and also feed a lot, but still they grow slow.
Bought them at 3 inch a year ago. One of them is 6 inch the other is 7. The third one never grew, but the third one is possibly a female that already laid eggs before.