I have a yorkie-cocker mix and she's 25 pounds, 7 years old and still looks just like a puppy. The only draw back is that her hair grows constantly since both yorkies and cocker spaniels have hair that grows a lot. So buy a great shaver and you'll do fine.
Since it's a mix breed there really isn't any way to tell. Could be 5 pounds or 55 pounds or anywhere in between. Checking out similar rescue and pound pups may give you some ideas.
www.petfinder.com home.comcast.netwww.wildheirlabradorrescue.org home.comcast.net
www.petfinder.com home.comcast.netwww.wildheirlabradorrescue.org home.comcast.net