It's up to you to decide what tank size you're willing to provide, and how much money you want to spend on food. Some of the smallest are garter snakes, hognoses, sand and rosy boas, and african house snakes. Those are all in the >3 ft. range. If you go up to the 6 foot range there's just a huge number of choices, even for a beginner. In fact, what your favorite color is could be a factor in your choice! (as long as it's not purple, that's hard to find. you're pretty much stuck with expensive boa constrictors if you like purple. green and blue are also tough but you have a decent chance with those).