Being able to adjust font size via key press is super handy. There is currently a vim script that has this behavior, and the script gives a good precedent. The script uses the following keybindings:
\\+ => Zoom Bigger
\\++ => Zoom 2x Bigger
\\+++ => Zoom 3x Bigger
(etc)
\\- => Zoom Smaller
\\-- => Zoom 2x Smaller
\\---- => Zoom 3x Smaller
(etc)
I suspect many users would find this a helpful functionality.