> One can C-x C-+ and C-x C-- to call text-scale-adjust,
> which increases or decreases the buffer text size, with
> C-+ or C-- to repeat.
> 
> Instead of repeating, I want to have a function that adjusts
> the scale so that the contents of the buffer fit the window,
> based on the longest line.  What can I do?

Dunno whether this corresponds to what you're
asking, but library `face-remap+.el' adjusts
the frame/window size to the scaled text size,
when option `text-scale-resize-window' is non-nil.

The particular non-nil value controls whether
the window-fitting is horizontally, vertically,
or both.

;; URL: https://www.emacswiki.org/emacs/download/face-remap%2b.el
;; Doc URL: https://emacswiki.org/emacs/SetFonts