Hendre i think your going by the chart size, even for a WC UJ its rarely seen them going 16", most that are in the tank are 10"-13" maybe 14" but thats only a few out there. Most CB UJ adult size in the tank are 8"-10" maybe 11"-12" out there but rarely too, unless your talking about the big three.
Most likely your sen will push 8"-10", that including my sons sen. Even my WC LJ there are no garantee that they will hit 20" its most likely 17"-19".
+1