Best Size to tell the sex?

Is there a weight range where it is best to be able to tell the sex of my gecko?

Generally speaking it is easiest to tell once an animal is an adult. Because of the variability in size, especially now that there are Giants, weight isn’t really a good way to assess sexual maturity. Age is probably a more accurate parameter.

Every breeder is going to feel differently about the age at which they feel comfortable sexing their leos. I personally prefer to just wait until they’re mostly mature (~1 year), but most breeders want to sell their geckos far before then and will either sell as (edit: unsexed), tentatively sexed, or get really good at differentiating early.