First, be prepared for the "teacup" backlash. To put it shortly, "teacup" is a term used by pet stores and backyard breeders for poorly bred runts. Perhaps you just meant "toy". "Toy" is the term used for the smallest AKC recognized dogs.
If by "adoption" you mean actual adoption through a shelter or rescue, good for you for taking a homeless dog on. If by "adoption" though you really mean purchasing, do NOT purchase a dog that someone is calling "teacup". A breeder who uses those terms is nearly always a backyard breeder or puppy mill.
As for health, again, "teacups" are prone to a strew of health issues because they are the runts being bred with runts. In general, their life spans are shorter and their medical bills longer.
Agression is something that will be determined by your dogs individual genetics, and the training you put into it. As for size, anything labeled "teacup" shouldn't grow any larger than five pounds.