Sizing for press on to sell

I want to sell press on nails online and in person, but I am confused about the sizing. I see a lot of people post videos about selling nails in booths and people just picked them up in person to buy them and leave but I wonder how do people know what size would fit if they aren’t just trying them on themselves?

I know online people often have sizing kits or have people message them with their own custom size.

Overall, I am just wondering how this all works in terms of size since everyone is different.