For collars to fit 'newborn' puppies as a guide I would recommend...
Small Breeds 5 - 8 inches
Medium Breeds 5.5 - 9 inches
Large Breeds 6 - 10 inches
Extra Large Breeds 6.5 - 11 inches
For the best fit it is always advisable to measure around your puppies necks, as each litter can vary in size.
Some times puppies will outgrow the collars that fitted at birth so a second larger set may be needed, I always try to stock the same colours so they can be purchased at a later date if necessary.
More Available Sizes
7 - 12 inches
7.5 - 13 inches
8 -14 inches