You would have to create a UNIQUE product for every possible permutation of size and thickness.

I have the exact same issue. openERP is not really setup to have unique parameters for each item of inventory. It is SKU based rather than a serialized inventory system. (by serialized I mean a list of products with a number infinitely variable parameters, not serial numbers.)

I am not aware of a custom module that can add the functionality, but I am working on something myself.

If someone has developed a customer module that adds the ability to assign things like length/width/thickness to a product, it would be awesome.