For some reasons I did a square invisible player and pinned some objects on him.
One of them was a circle with Physics behavior, where have that circle collision mask.
I used the circle to smoothly the detection between the player and some physic boxes on the screen.
Following this philosophy, you'll be able to reach what you want mixing it with the other types of collisions in your game, and of course, if you change the sprite size, the circle also change.
A long discussion was made about the circle polygon, on Stencyl community, and the result was No, they didn't implemented it because of the issues around the Hardware usage and performances.