Use the same hook size (and yarn weight) you used to make your puppy or go up a size.

I used a 4.5mm hook to make the overalls pictured and a 4.25mm hook to make the puppy.

I crochet fairly on the tight side and the overalls fit perfectly. If you crochet on the loose side than use the same hook you made your puppy with

Free Puppy Pattern here
Free Overalls Pattern here