Emacs: Set Minibuffer Font Size
There is no easy way to change font size just for the minibuffer.
Set Default Font Size by Command
you can set default font size globally, and the minibuffer will also have that font size.
Increase Font Size of Just Minibuffer, One Time Only
when cursor is in minibuffer, hold Ctrl key and scroll mouse wheel.
the command called is
mouse-wheel-text-scale