It's hard to tell really until you meet the little pup. They all come in different shapes and sizes so no one size fits all collar exists. They have some adjustable ones though. My little one could fit into a collar until at 4.5 months because she's tiny. And even then she only wears it for show because it has her tags on it. Also with frenchies and brachycephallic dogs it is recommended that you use a harness instead of a collar on them because putting pressure on their necks is a health hazard because they are smooshed face. If your little one is on the smaller side you can try buying a cat harness that's what I used on Nellie when she was younger and smaller.