Labrador retriever + small backyard?

i love labrador retrievers and i am 100% set on a lab. The only problem is i have a small backyard. I do not have a small backyard but it is not properly fenced in so we got some gates to have a small portion of the backyard safe. I have a Chi mix so its no problem for a little dog - but it would be too small for a Lab! Do you think they'd be fine if sufficiently exercised? If not, I could maybe expand it if necessary.