Adding custom fields to variations

Here's what I want to do:

I have 3 choices:
13" Laptop
13" MacBook Air
15" Laptop

I want a input box for Make and Model to appear when you select 13" Laptop & 15" Laptop. Is this possible?

Hi,

I think that the best solution will be to use 2 characteristics for your product :
- Make (13",15")
- Model (Laptop,MacBook Air).
Hope this will help you a little.

What needs to happen is if the person choose 13" Laptop or 15" Laptop the customer can type in their make and model. The client does custom bags and needs that information so they can look up the dimensions need to make the bag fit the laptop.

My bad, I misunderstood your question . I think that you'll probably find your answer there :

Yes that's possible. You can do that by creating custom fields of the table "item" (the field will be on the product page) or of the table "order" (the field will be on the checkout) via the menu Display->Custom fields in the Business edition.

Will it show up on every product, or can I select certain categories?

Yes you'll able to do that by putting all the products where you want your customfield to be displaid in a category, and then by restricting your item customfield to this category.