valve trim is (usually) defined as the combination of the wetted parts of a valve.

I can find example values of valve trim sizes, but cannot find a definition of that size. Apparently it is a distance, diameter or length, but which one?

 

It is diameter of the port or seat.

You will find trim that is the full diameter of the valve or a reduced port size.

Check the Cv charts for globe valves. The Fisher Control Valve Handbook is a good reference:
http://www.documentation.emersonprocess.com/groups/public/documents/book/cvh99.pdf