Boas are cool. I actually held a BCI for a couple years for an acquaintance who was in jail, and I really enjoyed the snake.
Proper Food Size for Young & Adult... P/K.. - a baby should be able to take an adult mouse, and they're pretty easy to get on prekilled/frozen-thawed.
Proper Humidity - not really high, 60% or so. Too dry = bad sheds.
Ways to keep humdity in large enclosure - stay away from aquariums with open/screen tops. Go with a DIY enclosure or maybe one of those prefab plastic ones by boaphile plastics or Vision. If it has to be an aquarium (ie it's already bought) give the boa a humid hide box, something like a rubbermaid container with the top on and an opening cut in a side, with damp paper towel inside.
Enclosure size for young to feel safe - that doesn't seem to be as much of an issue as it can be with ball/royal pythons for example. A smaller enclosure is fine for the young boa but I don't think it's necessary; it will make it easier for the snake to find food if you're not tong-feeding.
How large of water dish - I always go with something big enough for the snake to soak but some don't. Something heavy to avoid tipping.
Temp. Of Hot Spot / Temp. Of Cool Side - I went with mid-90s on the hot end and mid-70s on the cool end. I found if the cool end was any lower the snake never went there, ditto if the warm end went over 100. Hides on both ends.
Personal Experience - the boa I was keeping didn't have as strong a feeding response as other snakes I've kept (granted those "other snakes" were retics, burm, and kingsnake). It was a very calm snake, not jumpy/nervous at all. By the time it reached 5' long it was eating F/T adult rats without trouble. It was near 9' when I gave it back and eating 2 large rats weekly; I never felt the need to move up to larger prey like rabbits, chickens, or guinea pigs but that's just personal preference on my part I guess, I just feel feeding 2 smaller prey is preferable to one large prey item. It was an excellent shedder, especially compared to the retics I've had. You didn't mention substrate, I used cypress mulch for most of my herps and it worked well for the boa. The boa wasn't agressive at all, never struck, but then again neither were my pythons.