autoit and such can force it smaller but it will still render at 800x600. the method they are using in fullscreen windowed mode renders it at the DESKTOP res no matter how small you make it. so if your desktop is 1920x1080, and you force the window to like 640x480 it still renders the 1920x1080 into that small tiny box.
me and a developer friend have tested this extensively. we already had the thought of doing this and getting 100 clients going on one pc lol but it doesn't work. the best you can do is normal window mode at 800x600, but force them smaller to save space and tile them etc. set MAX FPS in game to like 10-15, and all low settings in D3prefs. DB seems fine at game FPS of 10, any lower and it buggers up a bit.
the fullscreen windowed mode DOES NOT WORK for using less resources.