Nobody knows the max size of that bichirs yet as it's still under the debate if they it's a new species, sub-species, variant or a hybrid.
You need to know that no one exactly knows the max size of every bichirs right now even those that are already described, as many are popping out that the size are bigger or smaller than what people previously claimed. So when it comes to lower jaw species, expect one to get up to 20"+ in aquaria.
BTW, just Google Polypterus sp. "Dabola" and you'll see some pics that came from Infblue and Toyin himself.