I don't know if this has been suggested before, but when you make the frame wider, the flash "jerks" a little (it isn't really smooth). I would assume that that was because there is some delay in the resizing of the movie. However, I've seen situations where a box of colour resizes smoothly, and eases in and out. My idea is that when you click that button, a black box fades over the area, covering the flash, and then that black box expands smoothly and then the movie would resize (this would only need to happen once) and the black box would fade out. Either that, or it could become a little border around the movie. I think that that would make the system feel even more slick than it is at the moment.
Brilliant job by the way. Newgrounds continues to surpass expectations.