All boas are constrictors, and pythons are not boas. They're all boids, but pythons lay eggs and I think there are some skeletal differences.
That said, I'll assume you want either a python or a boa. Most carpet pythons only get between five and six feet and aren't too difficults to care for. Ball pythons are small and easygoing but notorious for going long periods without eating.
Now, since everyone who replies is going to have a favorite snake and will try to get you to get one of those, and since I'm no exception to that rule, get a cb baby blood python. They're a little harder to care for, but in a few years you'll have a big (girthy; they rarely make it to six feet), fat, lazy, and beautiful snake. And if you treat it right, over time that nippy hatchling can become almost dog-tame.
I've never kept boas though, so I'll leave them to other people.