I have a yorkie. He's now nearly 9; I got him as a puppy when dd was 4.
Like all little terriers, he is an absolute little sod 
. He's a tiny dog with a HUGE attitude. He literally took 4 years to house train, and still pees up the open back door now and again just to remind me who's boss.
They're not particularly recommended as ideal pets for small children, but I made a point of taking him up to school as a puppy, and as such he is the most sociable little dog I've ever come across.
As far as size, he's particularly tiny - he weighs 5lbs - but most yorkies I've seen have been bigger/chunkier. It certainly doesn't stop him squaring up to the local rottweilers threatening to teach them a lesson though!
He does make an excellent watch dog and is incredibly protective. And he likes nothing more than cuddling up on my lap for a snuggle. As far as exercise goes, he's happy running round the garden after a ball, but then will happily go on a 15 mile hike through the countryside.
Any more questions, please ask!