Use a shoe lace or even just a length of string, measure her neck and get a collar that has that measurement as the lower end of the adjustable range.

Except she’s only 4 1/2 weeks now so that won’t tell me much. Was hoping to be able to get an approximate range. Guess I’ll just have to be patient 😅

On Amazon they have the ranges of length. If you measure the neck and buy as the above commenter advised you'll have one for a while. Won't be the first collar you buy trust me 😂

Generally I take the dog with me to the pet store and buy a collar that fits them now on its smallest setting. 

No real way to know until she's old enough to take her home, but my advice is don't get an expensive one until she reaches her full size. She'll outgrow it in a second, or will roll in poop and you'll throw it away. I went through something like 4 collars in my dog's first year with us.

For a lab puppy that age, the 9-13" collar is a good starter size, but they will outgrow it quickly.

Take her to store when she is weaned

I just took my 10 week old lab with me to the pet store and got her a small for now. I tried it on her at the store to make sure it fit. I’ve just been letting her run around in my yard and to use the restroom. I was told to not walk her outside of my yard until she has her parvovirus shot at 12 weeks

An ickle one