The easiest way to get The Powder Toy in fullscreen is to make a shortcut to the .exe file, bring up properties to this shortcut, and at the end of the target field add "kiosk" without quotations around the word. the original quotations should still remain on the main target. This is used to get many things in fullscreen at electronics stores where they showcase programs on computers. This will automatically set the resolution to the size that the application is running on. It automatically reverts to previous settings the moment the file that is run with the kiosk end note closes for any reason. I do not recommend trying to change display resolution while you are using this technique.
Example: "G:\Misc\Powder\Powder Toy.exe" kiosk