I want this too, and part of it should be simple.  Given that I have had a never empty backlog of PRs to review for 2 1/2 years, it got forgotten.  There was also a tk problem that seemed to block implementation.

Specification for hotkey: Cntl/Cmd - -+= decrease/increase font size within limits in the current window only.  (The current limits for turtledemo are 6 and 100.  Change this?  The keys are fixed and cannot be customized. Keys have no affect on IdleConf.  Setting font size in dialog continues to set font size for all Windows, whether individually set or not.  Should work same for editor and text (read-only) windows.

As near as I can tell, these key combinations currently do nothing.  I presume they are not bound in the built-in keysets.  Custom keysets are always subject to interference when we add new key bindings.  (I don't know that the custom key dialog checks against such fixed builtins. Another issue.  Checking custom keysets on import is likely a third issue.)

Implementation for hotkey: The desired behavior is the behavior of turtledemo, so the same implementation should work.  It is annoying that we need to add it separately for editor and text windows.  But refactoring to create a common base class would be a lot harder. 

Mousewheel: Currently, in turtledemo, control-wheel changes size on all systems.  

Raymond: Should it be cmd-wheel on Mac?

In msg226328 I reported a tk issue blocking both this and line numbers.  We now deliver 3.7.4 and 3.8.0 with tk 8.6.9.  We have line numbers because I no longer saw the glitchy behavior I remember.  Nor did I see any issue with font resizing with turtledemo.

I may include the module browser, etc, with this issue.  Dialogs would be another issue.