The woman at the shelter was quite wrong. Her job is to match a dog and adopter to ensure that that dog never has reason to come back to that shelter or any other shelter. If you are not confident with big dogs or the more dominant working breeds you are very wise to be clear about that up front.
I love big dogs, but I don’t, on the whole, enjoy Labradors. No good reason, I’ve met many a nice Lab, but they’re not the dog for me, on the other hand, I’ve lately fallen in love with Mastiffs. Doesn’t mean I’m a bad person because I prefer Mastiffs to Labs, it’s just a choice. It’s like the husbands of my friends, many a nice man there, but given the choice I wouldn’t marry any of them. 
I also think, that as you’re almost in the position of being a first-time dog owner, you’re being very sensible looking for a dog without too many breed-related issues. Pit Bulls can be wonderful dogs but they are inclinced to be dog aggressive, and you need a level of experience to manage that.
Dobes and Rotties are bred to be working dogs, they need lots of training to keep their minds active, again, maybe not the right sort of dog for a new dog person, although in the hands of the right person terrific dogs. Chows, well Chows are their own people, and I think you’re either a Chow person or you’re not.
If you had fallen in love with any of these breeds and were willing to put in the necessary work that would be one thing, but you were clear that you don’t want one of those dogs, and you do have a good idea of what kind of dog will suit you. Knowing what you want is a good start to finding a dog you can live with for the next ten or more years.
Don’t give up on the shelter idea, but find a better person to deal with…or if you must, deal with the individual you spoke to, but be willing to be firm. I sometimes think that shelter workers see so much of the worst humans can do to animals, that they tend to look on humans with a somewhat jaundiced eye, even the ones who are the good guys. Forgive her and keep looking for your perfect dog.
And if I might put in a small plug for Whippets, a personal favourite breed of mine. They’re kind, affectionate, curious, intelligent, independent and gentle. They like to run, but equally enjoy curling up on the couch or your bed. They tend to be healthy, need minimum grooming and are about in your size range. I know some people don’t like the way they look, but if you do, they’re a very sweet dog. There is sure to be a Whippet rescue around if you’re interested.