I edited an armor piece, changed:
Armor Rating: altered the number
The unnamed value next to weight : options are light, heavy, none, so i assume it sets the armor type
Keywords: I deleted the old Armor type, added new one
ID: I changed the ID
[OK]

this appeared to create a new object under the object editor window
but this new piece has a size of 0, the original had a size of 10

*Addendum*--> size field changes to 10 when i preview the object and than close the preview window

Question

What is the size field for?

another question: did I create a new object, which has no effect on the old?

The Size attribute's merely the radius of the object mesh's bounding sphere. It's calculated the first time the mesh is loaded into the render/preview window. And yes, it appears that you have indeed created a new object. Selecting 'No' in the confirmation dialog should rename the existing record's editorID.