How can I simply add a class to a layer in Photoshop CS5? That class should hold properties like "fill color", "size", etc.
I'm very new to scripting Photoshop so please bear with me =)
You don't. The classes that are provided by the applications you are working with aren't extensible. But, what you can do to get around this is to create a record making use of Applescript's dynamic typing.
The records, in turn, could be held in a list that you can maintain along side the layers. It isn't elegant, but it should get the job done.