There are lots of calculators out there if you do a quick google for "how big will my puppy be calculator" or something along those lines.
Take the results with a grain of salt though. You can estimate, but every dog is different. Knowing the weights of the parents can give you a better idea, but still just an idea
I would guess your puppy will end up on the big side of medium or small side of large. Probably somewhere around 50 pounds, but without knowing breed it's harder to guess.