I have a question about a skin for my laptop?

ok so i made a skin on skinit.com for my new laptop and i was just wondering how to tell what size the skin is. Cuz like laptops come in all kinds of different sizes. Mine's a 15.6'' and i don't wanna order the skin and then have it end up being like a 17''. How do i tell skinit.com that i want the skin to be a 15.6''? please help. thx.