There are dwarf boas that stay in the range of 3-5 ft, who are plenty large enough to take a baby rabbit but not too big for one person to handle alone... I've seen adult male Nicaraguan boas that are just about 3 ft and full grown. You have Nicaraguans, Hogg Islands, Caulker Cay, Sonoran, Honduran... Okay, maybe just easier to link you to a site like Vin Russo's BCI page
for a list!
It's rare for any of these dwarf localities to get over 5 ft, and even then you're just talking about particularly big females (sometimes just big, sometimes heavily-fed, even sometimes of questionable purity--it's very common to see Hogg Islands crossed with standard Colombians).
I could be mistaken, but I believe male Dumeril's boas stay in the same range (4-5 ft)? So they could be a good choice, too.
On enclosure sizes, everything that's been talked about so far (except maybe the Florida kings Beth mentioned) will need at least 4 ft of space. Carpets especially need a lot of space, as they are incredibly active. If your space limit is a 55 gallon tank, you might want to look at something different than a carpet.
You talk about only having space for a 55 gallon tank, but there ARE tanks of the same length that are MUCH more appropriate for a boa or medium-sized python. I personally hate
55 gallon tanks because they're so narrow. Really, too narrow for the majority of land-dwelling animals you might house in it (i.e. bearded dragons, boas, and pythons). I've found that the 6 inch difference between a 55 and 75 gallon tank is amazingly significant, and a 75 doesn't take up any more room than a 55. Actually, neither does a custom 4x2x2 ft cage... But that's beside the point.