Designers should not have to set the size of the mask effect

Opacity Masks should work as they do in Design programs.  When I set the mask I should not have to set the size.  

Example, I have a small red rectangle I want to use as a mask for a larger image, the red rectangle will resize to the size of the mask target.  To fix this, I have to create an Item with the red rectangle inside.  The item then has to be the same size as the mask target.

We should do this automatically.