Is what you describe above different than using "superscript" or "subscript"?
Yes, it is. "Increase font size" changes the size
of the font, not the vertical positioning; for example, a 10 pt font becomes an 11 pt font, 11 pt -> 12 pt, and so on. I assume that's not what you need for your equation editor problem.
As far as I can see it's not possible to assign a key to a custom superscript format (e.g. position 10% / size 100%), as the superscript default is a fixed 30%/66% and can only be changed manually. Assigning a key to the superscript function would always produce superscript with 30% position and 66% size.