I just don't want to end up with more dog than I can handle, I guess. :') I don't have much experience with larger dogs. The heeler mix was around 30 lbs, and my last dog was a chihuahua. I don't want to be one of those people that gets a large breed, then dumps it in a few weeks because they can't handle it.
From my experience, size isn't the biggest factor with how hard to train or handle a dog is... some of the larger breeds are actually more laid back than some of the smaller ones. Your Aussie example or a Border needs a lot more attention than many labs, and many goldens are so mellow you have to try to get them to get up and go. I had a tough lab, and he was only hard because he was thinking sex all the time, neuter him earlier, and he would have been a dream. If you want an easy temperament, try a golden from show lines (field dogs will by higher drive) I showed a lot in obedience, and Goldens seemed to come out of the box trained and obedient. Heh.. I used to sortof look down my nose at them, and the Sheltie people since they never had to work on it at all...
Again, you can get a feel from the parents how 'intense' they are, and how 'hyper' they are.
You are smart to worry. Bubba weighs 96 pounds and is strong as an ox.
He's pretty well mannered, but if he really decides to go somewhere, he could pull me down and I'm rather small. I use a prong collar which slows him down (a little).
Just be sure he's well-trained and you'd probably be okay.