> It's called from xdisp.c as
> 
> 
>    if (FRAME_MINIBUF_ONLY_P (f))
>      {
>        if (!NILP (resize_mini_frames))
>      safe_call1 (Qwindow__resize_mini_frame, WINDOW_FRAME (w));
>      }
> 
> 
> which represents one of our ways to call a Lisp function from C.

Thanks, for some reason `debug-on-entry` did not invoke the debugger
when I tested it.

> Right.  And if you have any questions, ask here immediately.

I never dealt with the C side of Emacs and you suggested to offer the 
new function as first customization choice for `resize-mini-frames`.
I guess this needs to be done on the C side as the variable is defined 
there?

I have attached the patch I have worked on. Let me know how to proceed 
and thanks for your help!