The only thing I know for sure is upwards of 18 inches. I had an 18 incher and the guy I sold him to said he's still growing and is currently about 20 inches.
My biggest is over 18" and still growing and ive had her from 4",i know tom500 on here had a 20"+ one and there is a 24" one in one of the recent threads in here or the poly media lounge hth.
18"-22" would be average captive breed. Wild caught can pass 22" but that would depend on the owner's care as it may just stay 18". At 18" where the growth rate slows down.
my ornate is very calm and actually lives with a ray happily and most bichirs mouthes are small so buy baby ornate your only regret will be they are not as active a senegal