I work at an animal shelter so I talk to people looking to get a dog every day.
Here's the number one mistake people make when choosing a cat or a dog, and it's the reason people end up unhappy and returning or re-homing their animal:
They choose based on looks.
Sometimes that means they say "ooh! Yorkies are SO CUTE! I HAVE to have one!" Yorkies are cute, yes, but notoriously difficult to house train. The average person does not have a lot of patience for cleaning up urine in their home over and over and over. http://blog.yorkieshampoo.com/housetraining-your-yorkie/
Sometimes that means they go for the dog they feel most sorry for. "Oh, look at that big dog that's about to be euthanized.Look how sad his eyes are. It will feel good to rescue him and give him a loving home."
Then they get the dog home and find out that without consistent, focused training, big dogs can chew up furniture, knock people over, leave scratch marks from jumping on people, and they need you to take them for long walks every day, rain and shine.
Because choosing based on looks can be such a huge downfall, I recommend that when you visit the shelter, do not walk through the kennels. Call ahead and say that you'll be there at a certain time and that you would like to look at dogs one at a time who have a certain attribute. We would have no problem doing that at our shelter.
For your situation, I would say medium sized dogs, and don't be afraid to look at a dog who is five years old or more. Old dogs are great couch buddies :) I would ask to see the owner surrenders first because those dogs have lived in a home before and the shelter might know a little about their history.
Good luck!