Most breeders will set a certain weight or age minimum(which ever comes first) there are quite a number of threads on this, the search function on the forum should be of use.
15 grams is the typical weight that breeders feel comfortable to ship. At this weight, geckos are less fragile and less likely to succumb to any stress from a change in environment compared to a gecko that is smaller. Many breeders also like to keep hatchlings til they are around 20-25+ grams to see how they develop visually. Sometimes a geckos potential isn't really seen til 20+ grams.