Rounded Rectangle ..Corner Radius???

Masks are vector paths not parametric shapes so there is no corner radius you can adjust. You cal select all mask points and then press Ctrl/Cmnd + t to bring up Mask Transform, hold down the Ctrl/Cmnd key or Shift + Ctrl/Cmnd and resize the mask around it's geometric center and then use mask expansion to generate symmetrical round corners. The Mask path will still be square but expanding the mask with Mask Expansion will give you a radius on the corners. The amount of expansion is equal to the radius. In this case I have created a mask with a 78 pixel corner radius.

Another option would be to use a shape layer and the rectangle tool, adjust the size of the parametric shape using the tools shown in my first reply to this thread, then use the Shape layer as a Track Matte to create a mask or right click on the Rectangle Path and select convert to Bezier, set a keyframe to make sure you have the path selected, then copy. On the layer where you want the mask select the mask tool (g), click anywhere and paste.