The size designations (13", 15", etc) refer to the size of your screen if you were to measure it diagonally.

These skins should fit Mac laptops well; You may need to trim the skin to fit it perfectly to your PC.

You will want to leave some margin around the edge of the top of your laptop for an ideal fit - you don't really want to wrap the skin over the lip of the top of your laptop.

The dimensions of the skins are as follows:
12": 10.25" x 8"
13": 12.125" x 8.25"
14": 11.875" x 9.125"
15": 13" x 9.5"
17": 14.75" x 9.75"