As previously discussed each manufacturer has different sizes.
So a size 8 in one might be a size 10 in another. You cannot have different sizes on your site dependent on which manufacturer you source from, similarly you cannot build out your own brand if you only source from 1 manufacturer to keep the sizing consistent. Further some women wont wear a size 14 even if that is their correct size because they think it makes them fat, so they will only buy brands that have a size 14 marked as a 12!
In short if your just starting out and don't have experience of fashion that it not the place to start. Pick something that is less likely to be returned.
If you don't believe me and
@Billy Batts then go ahead and do it and learn for yourself.