Until the mods get image sizes under control, I made a userscript you can use with Tampermonkey or other browser extensions to reduce the maximum width of images.

// ==UserScript==
// @name Resize Images
// @namespace http://tampermonkey.net/
// @version 1.0
// @description Resizes images on ProtonBooru so that they actually fit on screen
// @author TheOrangeSauce
// @match https://proton.booru.org/*
// @grant none
// ==/UserScript==

(function() {
'use strict';
var image = document.getElementById("image");
if(image){
image.style.maxWidth = "80%";
}
})();