We have quite a lot of members here with mixed collections and a lot of wonderful boa keepers so you're in the right place. My family currently has 13 ball pythons of various ages as well as a red tail boa (BCI) and a milksnake.

Not that i'm afraid of being bitten, but are these snakes aggresive?
Well as you know any snake can bite but boa's tend to be pretty laid back snakes that can put up with a lot more handling than most. They are famous though for their strong feeding responses so come feeding day don't be slow with the rodent and be smart about getting between a hungry boa and it's dinner. Boa's tend to have quite a long strike range so if feeding f/t a extra long set of tongs or hemostats is a good thing with them.

How large do they usually get? and what do they end up eating when they get to their full size?
Females generally are larger than the males (I believe size quotes have already been given) so it's best if you can have this boa properly sexed by a herp vet, a breeder or an experienced hobbyist so you know up front what you are going to be dealing with at maturity. As far as feeding boa's unlike ball pythons aren't terribly prey specific so mature ones will often eat large rats, guinea pigs, small bunnies or the like. Most don't care what's for dinner as long as it's furry and warm. Always be mindful however to not over-feed or powerfed as a fat boa is an unhealthy boa.

is their habitat pretty much the same as the Ball Python? or is it much different?
I keep our boa slightly cooler than the ball pythons but by only a couple of degrees so basically very close as far as temps and humidity. I do find the boa really enjoys burrowing so we keep him on a thick aspen bedding and of course his big hide on this warm side of the enclosure. They do get larger than ball pythons so plan on moving up in enclosure size as needed.

Those are my main questions, i think they're gorgeous, but need to find out everything before i invest.
Smart potential owner! Good for you. They are great snakes but are on the larger size so it's best to decide up front if you want that so you don't have to eventually rehome the snake. They are nothing like a burmese python or that sort of size but still a substantial, strong snake. Personally I love boa's and can't imagine our house without one in it.

He then went on to say as they got bigger, they started to eat small chickens, and or small rabbits? Now i had thought that even to their full size they just ate Rats? Is this true?
Actually ball pythons can do fine their whole lives on either mice or rats and never should require anything larger than a small rat (maybe two for a big breeder female). Lots of us here have ball pythons that are supremely healthy and fit and have never eaten a rat but regularily eat multiple mice per week. Ball pythons are very prey specific and some will not switch from their prey of choice. It's all about appropriate volume of a well fed, well hydrated rodent whether that's mice or rats. I have both mouse and rat eaters in our collection and I'd bet you that you can't tell which is which.

Hope this helped.