My WC female has stopped growing at 17-18". She may have stunted her own growth by being stubborn and eating frozen shrimp, and not much of it, for the first year I had her.
My other upper jaw bichirs who started out smaller than her ate like hogs and grew at a decent rate. The palmas polli I bought with her was 2" shorter than her at purchase, and outgrew her in about six months.
Once she began to eat heartily, whe went from 7" to 12" in a few months. First year, she went from 5" to 7". That's my Princess. A stubborn obstinate ornate that does what she wants to, when she wants to. She's good looking for a fish so I can't complain.