Hmm, it didn't work out 3 times already? Sounds like you guys may not be ready for the responsibilty of a dog yet.
Have you been buying them as puppies? If so you've got to remember it takes time to get them house broken and alot of patience over the first few months for them to learn what good behavior is. Remember they just want to please.
As for size, there are lots of good medium sized dogs that give you the best of both worlds.
To chipsdad: about that tool for finding the dogs that's right for you, there is one site that i've found over the years that has something like that. It's not the greatest considering it only shows you breeder links after you input 3 variables but it's something like that. That's really a great idea though. Here's the link to that site:
http://www.puppyfind.com