Yes! As long as it's sprite based, it should be possible in SmileBASIC, especially an RPG since those require little processing power. As for size, again, yes. Programs have a very large limit, and if you did happen to reach it (Which is doubtful), you could just create another program to continue the game.

Lucky American.

Does this actually teach you how to make them or do you have to know how to program & code?

I think program and code.

is it worth it? 10$ is a good price but is it worth it?

If you're a coding expert, yes.

You don't have to be an expert by any means. When I bought Petit Computer, I didn't have much experience coding at all, and I learned coding through Petit Computer, along with fimding out how much I enjoyed it! I can't imagine SmileBASIC is any harder for beginners than PTC was.