Its really simple to edit the size of anything in Oblivion using TES, monster, item or scenery.
In TES editor, place the monster you want to make larger in the Render window at the location you want it. Then, double click it, and in the window you should see a "Scale" option at the bottom of the 3D Data tab. Just increase or decrease it to the size you want.
I hope this is what you wanted.
To easy