No way past 15 with hemipenes.
15 feet is the upper end of average for a male mainland Burm - NOT the absolute maximum size. Please do more research - and do not just trust one source for your information.

Still a 18 foot male Burm - is basically like a 7+ foot man - in that both are considered exceptionally large individuals for their species. But still very much possible.

It is often reported that as a species Burmese pythons don't exceed 19 to 20 feet in length. Those are based on the known "confirmed" reports of the largest known specimens kept in captivity (usually Zoos). However there are some "unconfirmed" reports of Burms which are about 23+ feet in length, and weighing more than 400 lbs.