The dog would get a knee in the chest. I wouldn't do it hard, but enough to force the dog to back off. If not that, then I'd go ahead and step on the rear paws.
Although my larger dog isn't that big, she's 100 pounds and used to be all muscle... very strong. I didn't put up with that, but there was nothing we could do to get her to quit jumping on people. We finally started the trick with the knee and/or the feet and it worked like a charm.
You want to be careful with smaller dogs though. I did it to the dog in my sig (about 45 pounds) and she flipped over.

I swear I didn't knee her hard! I just put my knee between the two of us, but she was coming at me so fast it flipped her over!
ETA: I'm very comfortable and confident around dogs... if I sensed in any way that the dog was being aggressive, I'd probably instead try to turn my back and appeal to the owner for help.