I have a cross between a black Labrador and a jack russel called Bitsy. From my experience, every one looks different, even bitsy's 2 sisters were very different, one was the size of a Labrador with jack russel markings, the other just looked like a black labrador, then bitsy was the smallest, about a foot from the her feet to the top of her head, the link above is a photo of her, shes about 12 years old in that photo, now 14, still quite puppy faced and very hyper, but the loveliest friendliest dog ever
What a Jack Russell Terrier and Labrador Retriever mix looks like is an adorable dog. It kind of has the lab fur and mixed coloring. It's ears are folded over at the tips. For a picture of one see: www.dailypuppy.com/dog/angel_10489.