1) Cronker, if you want a job as a dog trainer, let me know ;). Nice advice!
2) When I started off as an animal trainer I had a great disdain for small dogs. Now I think they're every bit as excellent as bigger dogs. In some situations, they're even better. Chihuahuas can be superb pets. They're smart, loyal and if well socialized as puppies not as "yippee" as a lot of them seem.
Cavalier King Charles Spaniels are gorgeous, sweet mellow dogs. BUT the US bloodlines are prone to a host of devastating genetic diseases. I can hook you up with a US broker who imports some great Irish bloodlines if you want.
Pugs are as sweet as can be and are low maintenance, Ditto Boston Terriers (a terrier in name only). French Bull Dogs too,
Toy poodles are ultra smart and easy to deal with.
More unusual breeds that might be good matches: Lowchen and Tibetan Terriers.
Now for the creative option: you may want to think about a retired racing greyhound. Greyhounds may be bigger than you had in mind, but they take up very little space. All they want is a cushy place to lounge for most of the day (many like being crated) and two 20 minutes walks. At the track they are crated for 23 hours a day so they do really well in apartments. Most have no need or desire to run. They're commonly known as "forty mile an hour couch potatoes". (Though, if you can arrange for them to run safely, it is a beautiful thing to watch.) Greyhounds are also incredibly soulful dogs. I volunteer some of my time with local greyhound rescues and, while I generally form some connection with all of the animals I work with, I have fallen in love with every greyhound I've worked with. If I could choose one dog breed to keep just as a pet, nothing would even come close to a greyhound. My hands down, low maintenance favorite.
If I can offer any other advice, please don't hesitate to let me know.
And please be sure to let us know as you decide.
All the best.