Hello notafish.
Well done for considering a rescue dog. Each dog is carefully tested, and as long as you exercise common sense, most rescue dogs settle well into new homes, glad of the second chance. If there are serious problems, rescue centres will accept a dog back into their care, usually.
Do you have young children? Are you out of the house a lot? Can you afford to pay for EVERY aspect of the dog's care? I know these might all sound obvious and a bit patronising, but it is just stuff that some people don't think through before mking the (BIG!) decision.
If you go for a puppy instead, make sure they are healthy, bred by someone who cares about the breed. See the mother, visit your puppy a few times. Good breeders won't mind this. Someone like Lankeela, Jules or Kita will tell you lots about buying from a breeder. Never EVER consider a breeder who is advertising 3, 4, 5 different breeds. Most likely a puppy farmer, and a potential nightmare.
Our first dog is a Collie, and we had SO much to learn! He had a raft of issues, and we were misled about him. So I am keen for any other prospective owners to know what to expect, and how you can be sure that you are getting what you think you are.
Dogs that are usually good with people range from Labradors (popular choice for a family dog) to Staffies, to Greyhounds, which I love, and lots in between. So either way you have a lot of choice within that. Small dogs aren't necessarily ideal, it depends entirely on what you want. And small dogs can sometimes require more exercise than some big dogs!
Can you be bothered to walk a dog morning, afternoon, night, for 15 years? Can you pay vet bills or afford insurance? Can you flea and worm your dog regularly, and afford their annual boosters? Can you afford the cost of getting them neutered/speyed? Can you buy them good quality food, lots of toys etc?