What I did was: 1) open Dimensioned Lumber in Furniture Library Editor, 2) Resized the particular piece, 3) deleted all other items , 4) merge with Dimensioned Lumber, and 5) save to Dimensioned Lumber by overwriting. I don't understand what you mean by export object to obj/mtl. Are you referring to the object that has been modified in SH3D, or in Furniture Library Editor?