Create a full frame size rounded square.

• Tools -> Roto -> Rounded square
• Enter hardness value

• A Roto node with a full frame size rounded square.

• Copy 'roundedSquare' folder in your .Natron folder.
• Add following lines to your 'initGui.py' file, where <path> is the location of 'roundedSquare' folder.

from <path>.roundedSquare.roundedSquare import *
NatronGui.natron.addMenuCommand('Tools/Roto/Rounded square','roundedSquare()')