Create a full frame size rounded rectangle.

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

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

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

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