Mmm, thanks for the suggestions, sjpt.
I think I didn't express myself well. I was meaning I wanted to size the actual display specifically, rather than the application's window, but you've given me an idea.
Perhaps I can create a document with an image at fixed dimensions — 600 x 800 — and use that to resize the ADE's display. I'll see if that works.
I know the point of CSS is to reflow dynamically, but I think it would be useful if ADE could be set to predefined sizes, like webpage apps can.