A general rule of thumb is 40 grams for females (can be a few grams less if she doesn't have a tail) and at least 12-18 months old and 30-35 grams for males and at least 12 months old. You should also check the calcium sacks on a female prior to breeding to make sure they are good and full. There are other things to consider as well--overall health, whether or not the geckos have been rested sufficiently if they have been bred before, etc.