The best approach is, if possible, determine the objects you want to use, open them in a grid, scale them and save as .tgo with the amount you have to scale up or down in the name; plant_plus2.tgo. For the really huge .obj's import them into the 10 m grid and source in cm in obj options before scaling with the grid. Sometimes it's un necessary to tweak further but y don't know unless you have the grid..