Determining how much of an area covered

I’ve got a player controlled square that’s emitting a trail renderer and I was wondering if anyone knows if there’s a way to figure out of much of the total area of the play field that the trail renderer is covering? If trail renderer won’t work, is there some other way to make it work?

If the player can only move on 4 directions and if each moves is constant, sort of like a big array of available position, or a screen and the player cam move from one pixel to another, the area covered in % is numberOfDotsCovered / numberTotal.

Now, if the control is more free, like a pencil or a drunk baby, that’ ssomething else entirely. Not sure about that one.