I may be wrong, but I would start here...
Code:
nUI_Options = {
["scale"] = 1,
["hScale"] = 0.75,
["vScale"] = 0.64125,
-- all the rest of your WTF settings
}
To make your UI narrower across the display area, reduce the value of hScale and visa versa. To make your UI shorter up and down the display area, reduce the value of vScale and visa versa.
You do have to log your character out (but do not have to exit WoW) between attempts to change the WTF file. So... log out, change the settings in your WTF file, log back in and check. If you don't like the results, rinse and repeat.