nUI does not set the size of the tooltips, WoW does. You can press "Escape" to bring up the main menu in WoW, select "Video" and then turn "Use UI Scale" on and reduce the scale until you get it to a comfortable size. Once you get the scale the way you want it for the tooltips, etc., then do a '/nui rl' to have nUI recalculate its own scale.
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