How to enlarge a square mask in Premiere Pro?

[RileyTheFox]

Hi. I am trying to mask a video clip to just a square (for chroma keying) and I can't enlarge the mask without losing the 90 degree angles on the edges. I want to keep it a perfect square but make it bigger. I've looked all over the place but nothing works. People say select a corner then Shift-Drag it but that does nothing. How do I do this? 

 

Thanks.