I suggest a harness instead of a collar if you’re planning on walking it. Puppies always tend to choke themselves not understanding the concept yet. It’s hard to guess the size of the puppy, but if you’ve already met him/her you can probably guess correctly.
A good website about sizing
Also, if you choose the route of a collar, make sure you can place two fingers between the collar and their neck. Puppies do not like strange, new things so he might scratch at it, but it doesn’t mean it itches just that he wants it off. Puppies usually like the leash even more than the collar.
Make sure you check your puppies collar often as he grows and make sure it never gets too tight.
Good luck with your puppy!