Ingernet research28" is the world record for a Dovii, in captivity it's likely to be more like 20-24" for a male, females considerably smaller. Where did you see 38"?
Jaguars can MAX at 21.5" (again world record), but 18" seems to be the max in captivity, with 11- 14" most common. You will not likely see a female over 11"
EDIT: Just realized a 240L is only about 65 gallons......yea, you might have a little too much for a 65g