I would consider a small Lab. Look at my post 33 - and look how short Dozer (the Black Lab) is standing against Patti's knees. If he didn't have all that show fat on, he wouldn't weigh much more than my girl.
Shelley is 20 1/2 inches at the shoulder, or 21 1/4 (depending on whether she was measured by a USDAA or an AKC judge!) and she weighs exactly 47 pounds as of last Friday. She is as tough as nails, has great endurance, and although she has that hot field trial temperament, she's very biddable and obedient. For example, she never pulls on a leash, ever (well, unless a squirrel runs by right under her nose).
I know some field trial dogs that are on the small size (Shelley's good friend Tori weighs 55 pounds and is no taller than Shelley - the difference in weight is due to her longer back (another field trial trait)) but your best bet for a small Labrador is to get a show dog with English breeding predominant (the English Labs are smaller than the American Labs) -- and don't over feed the dog!!!.
There are field trial Golden Retrievers that are small like Shelley - the show Goldens tend to be huge big dogs. Of course the show Goldens are the mellow ones. I thought about a Golden but couldn't deal with all the hair and the constant shedding.
Thanks, all of you; Sorry to run out like that! I think I'll go ahead and add Labrador to my list. As a previous poster mentioned, there are tons of them at the pound, so I'm sure we could find a smallish one that would suit our family. That might be easier than going through a rescue, anyway. :'P