Lots of these questions lately
As I understand it, there is no firm answer. each loft is going to have their own design which will make the sail slightly larger or snaller in total area than others. They'll vary by a few %. This i unlike a main, for example, that would have set dimensions. If you're shopping around just ask the loft for the specs on the sail so you can compare. If you're making you own... _then_ you need the specs!