Hey I wish I could be more help but there are hundreds of species of snake that hit that description you just gave. if you want boids (boas or pythons), some suggestions are any of the sand boas, children’s pythons, spotted pythons, hogg island boas, ball python, Amazon tree boa's.
then if you look into colubrid species there are several species of king snake and milk snake. Rats snakes, and of course corn snakes. That all fit your description some will become a bit longer then ball pythons but nowhere near the girth so still relatively small species.
Since it’s your first snake I suggest you look into corn snakes they are fairly small, very hardy and docile, they also come in an amazing number of different variations.
Try this site.
http://www.extremesnakes.com/ he has a good gallery to show you what I mean.
Devon