Good for you for giving this a lot of careful thought ahead of time! I would definitely recommend an adult dog for your family (at least one year old), because puppies are SO much work and SO difficult, plus you never know what you are going to get in terms of temperament. There are a lot of really great shelters in this area, with thoughtful adoption counselors (I'd recommend the Animal Welfare League of Arlington, the Fairfax County Animal Shelter, and the Washington Humane Society/WARL facilities specifically) - go there, be candid with the volunteers/adoptions staff about your needs and expectations re: energy level, grooming, size, etc., and see what they recommend. All dogs really are individuals - as a volunteer, I've met giant formerly-stray dogs who couldn't have been more gentle and laid back, and tiny pedigreed dogs that I didn't want anything to do with because they wanted to eat my face
The other thing to ask about, since you are new to pet guardianship, is foster-to-adopt opportunities - this gives you the chance to "try out" having the animal in your home before committing to adoption. Most shelters don't necessarily advertise it as an option, but it never hurts to ask, especially if you are doing it as a good-faith effort as part of the process of adopting. It may take multiple visits, but I'm sure you can find a dog that's perfect for your family!