I agree with Corgi (even though they're technically a medium sized dog). We have one (a cardigan not a pembroke which is what the queen has) and he's wonderful. However, if you're looking for a truly small dog I'd look into a different breed as mine is 32 pounds and solid muscle...that's not exactly small.
I'm very surprised no one has said this yet, I have a Japanese Chien he's about 10 pounds very loving and friendly, most Japanese Chien's are like that, they would make a great family pet if trained well. They are one of the easiest dogs to train because they were originally bred for the queen and circuses