Is a Lot of smaller Species of Snake you can Start with...
Spotted Pythons Are Great pets they stay small (within requirements of your mom) but they are not too bulky...
Kenyan Sand Boas are nice small stocky snakes...
Rossy boas are really nice too and is more and more morphs available now...
You can look in to African House snakes as well is few species available on market.. there are not that popular but they are not expansive either... They are easy to care for and grow to 30-36 inches... and they are look kind of like miniature pythons...
Males of some milk king snake species stay relatively small as well around 4 feet...
So you have a lot of choices...
Do your research is a lot of info available on internet... Do your homework.. print information pictures Show your Parents real facts about species, caging, behavior..and give them few options...
That way You show them that You are really prepared to own the snake and will be easier to convince them.
Good Luck![]()