Well, I don't know where to beging actually. There are so many things to be aware of when getting a dog! How much experience do you have with dogs, what's your living conditions, how much time can you afford to spend on your new companion, whats your lifestyle, and so on...
The first, and maybe the most important rule you should consider is, do not ever get a dog with higher energy level as your self! If you are lazy, get a dog that have equal or lower energy level that yourself, and also if you are an athletic/sporty person, the same rule. The second, how much experience do you have with dogs and how much are you willing to learn and give of yourself. The third... I don't know.
In my case: my first dog was an Alaska husky. A very demanding dog and not one for beginners. But she taught me a lot and awakened my interest for dogs and dog behaviour. If my first dog was an easy one, maybe I wouldn't be a canine behaviorist now...
Now I am a person that like to spend a lot of time outdoors.
So for my second dog I chose one with a high a energy level (because I also have it), a high prey drive and hunting instinct (because I will use my dog for hunting) but also one that is affectionate, social and likes to cuddle with me on the sofa. That's why I now have a Gordon setter, and not an Alaska husky.
Yeh and btw, big dogs are often the most calm ones. I have been thinking for myself that when I'm like 70 years old, I will get a Leonberger! They are enormous, and chill, and I just love them. But way too slow for me now!!!
My point is, it's not the size. It's the energy level of the dog that should decide if it's right for you or not.