Thanks! Both fair points. Right now zoom is the +/- buttons in the drawing's corner, or Ctrl + scroll (pinch on a trackpad), which clearly isn't discoverable. I'll make the plain mouse wheel zoom the drawing and give it more room on big screens. Will post here when it's live.

Live now: the mouse wheel zooms the drawing (scroll down again to zoom back out and keep scrolling the page), and there's a "Bigger drawing" button next to the zoom buttons that gives it the whole window. Thanks for the push!