what anne said....other than weeksii, ornate, and possibly teugelsi, most upper jaw bichirs have recorded max size that are within 1-2 inches of each other.
based on my observation, the 5 that are most likely to stay the smallest in captivity, due to slow growth or whatever, would be senegal (albino and normal), mokelembembe, palmas palmas, palmas buettikoferi, retropinnis (no specific ranking though)
as for the rarest, excluding special color morph, based on the number of specimens available in the hobby alone, it'd be teugelsi at the moment, no question about it.
If we also take into account how long the fish has been known, i'd say the rarest would be ansorgii (very limited in the hobby, but known for about a century), and we've only known about teugelsi for several years.