The placeholder is the see through box that follows your mouse in the Minigame Creator section. Whenever you click on a surface, it will place whatever object you have selected (either a shape, coin, or key).
Default Controls:
Up Arrow: Increase shape height
Down Arrow: Decrease shape height
Left Arrow: Increase shape width and length
Right Arrow: Decrease shape width and length
Up Arrow + Ctrl: Increase length
Down Arrow + Ctrl: Decrease length
Left Arrow + Ctrl: Increase width
Right Arrow + Ctrl: Decrease width
Up Arrow + Alt: Set angle to 360
Down Arrow + Alt: Set angle to 180
Left Arrow + Alt: Rotate towards the left
Right Arrow + Slt: Rotate towards the right