Ornates in the aquaria can easily reach 20" wiith time, but anything larger is very rare. Most Captives stay closer to this 18-20" threshhold.
As for Wild Caughts. Based on hobbyists of old whom dealt with people out in the fields catching polypterids (fishermen). 24" +/- does happen, but not much larger than that. Or...... possibly monster specimen sizes haven't been paid attention to due to human consumption purposes.
The majority of the 22"-24"+ specimens are overseas. Due to english not being the first language. Conducting a Google search "largest ornate bichir in the world" will not pull up, lol.... They're featured on occasion w/ facebook, personal blogs in their language, Instagram, pictagram, etc., or not at all.
* Pic 1 - this looks to be 22"+ at least. The fisherman spaced out fingers likely equate to 3". Run that long the ornates body. Then this specimen minimally becomes a 22-
24" fish imo.
* Video 1 - belongs to Jerry Teo. Filmed 2009. Ornate(s) in the video are 62cm (24")
* Video 2 - is when the ornate sadly passed a year later. The polypterid is measured and clearly seen it is 62cm at the end.
So, after viewing the materals below. What are your opinions? For me It is likely that they can reach larger than 24"+ or closer to 28" number or even 30" in an optimal environment. Just my opinion. 
