I don't have NifSkope in front of me, so I can only approximate the steps from memory (so if I'm wrong, it's not my fault
).
Basically, you want to open the mesh in NifSkope.
Find the main section (click on it in NifSkope's render window).
The tree in the left panel will expand, showing you what you just clicked.
Right click that item in the tree (it should be an NiTriShape, but might be an NiNode), and click edit transforms (I think that's the option).
It'll open a box with a few inputs for position, angle, and scale. Increase/decrease the scale, then hit ok.
Go back to the tree view, right click and hit apply transforms. The render should change.
If this makes other things look off, you can go to their transforms and change the scale and/or position to match.
Don't try to edit the transforms on the main NiNode, nothing will happen in Morrowind. I'm not sure if you can edit NiNodes at all (and have it effect things), so you might have to edit each NiTriShape.