Toonstruck runs in ScummVM behind the scenes, so ScummVM hotkeys work. You can see all of them in the README at scummvm.org, but the most useful ones are Ctrl-Alt + and - to increase or decrease the scale factor, and Alt-enter to toggle between full-screen and windowed. So if you have a tiny window in the middle of a black screen, hit Ctrl-Alt + once or twice. It still won't fit perfectly on a modern monitor, but it'll be much better.
