Then all bets are off!
I'm sure there are some vets who are pretty good at determining what breeds are in a mixed breed dog, but I have done a number of DNA tests on mutts, including my own rescued dog, and don't usually get very close! For a puppy with pretty big feet or a smooth coat, a lab is a good guess. This is mostly because there are a lot of labradors running around--the odds are good! If the puppy has a fairly long nose
, a collie is a reasonable guess, but a Sheltie looks a lot like a collie, but is about 30 pounds lighter.
I guess what I'm saying it that, unless you do a DNA test before you adopt him, you really don't have any idea what he is mixed with. If he weighs over 15 pounds at 8 weeks of age, he will likely be a pretty good-sized adult, but if he is 10 pounds or less, he probably won't get above 50 pounds full-grown.
If you need a dog of a certain weight or less for an apartment or something, then I would look into getting a purebred--they have many purebred rescues where the pups don't cost too much.