Initially I was thinking of making a plugin for this and for that I made a post with 'Development' as its category here Keyboard Shortcut to increase/decrease editor font size - #11 by makb
But after reading the doc for Plugin API and discussing it in above post I think that maybe it is not possible to change Joplin's default Settings such as 'Editor Font Size' using Plugin API.
So I think it would be good if we have a default shortcut to increase/decrease editor font size.
I am willing to work on this if we agree that this is good idea.
Thanks.