Method 1

Step 1: Open The Powder Toy.

Step 2: Click on the tick icon (Simulation options) on the bottom of the window, then check the box that says "Fullscreen". You can also check "Large Window" to have the window be twice as large, but not completely fullscreen.

Step 3: click "Okay".

(Does not work on some older versions)

Method 2

Type this into the console: tpt.setwindowsize(1, 1)

To get out of fullscreen, type tpt.setwindowsize(1, 0)