I'd look into CAs and Nics, they tend to stay a lot smaller than other BCIs and still come in nice morphs. Hogs are also "dwarf" boas, but some females have gotten just as large as Colombian females.
If you don't have the room for a larger enclosure (I'm look at at LEAST a 4' cage for my male and 6' cages for my females when they get larger) then I'd shoot for a smaller species to start out with. I doubt you'd end up with a monster boa but better safe than sorry!
Hatchling growth depends on the snake. Some have BIG spurts, some are slow growers. For example, my jungle male I got back in May of '11 was at 217 grams when I got him and of April 8th he's up to 1400 grams. Big spurt IMO. But the little albino girl I got in November of '11 at 132 grams is only at 214 grams as of April 8th 2012.
Adult size is on personal opinion, I think when the snakes becomes three+ years old it's considered an "adult" - my 5' male right now I would still consider a "subadult" as he was born in June '10.
I do once a week for the younger ones, then move to 10 days when they get on mediums (though 2-3 weeks is good too) and will probably scale back feeding even more when they get on larges. Good to avoid obesity in boas: they easily get fat via overfeeding.
I keep their humidity at the standard 50% with perfect sheds, and give them 88-90 hotspots. I noticed my boas tend to like cooler temps over warmer ones like the BPs.
And because I can....some pics of my guy and girls
