You're close, but the first thing you have to do is MAKE SOME SPACE. Seriously, you can't see your VP's and you're working digitally. That ain't right. Expand out the canvas on the left, right, and bottom sides so you can see where this all ends up.
Now, to figure out the edges...again, you're damn close. All you have to do is plot a line from each vanishing point to those lines emanating from your light source...where they intersect are your edges.
I should point out though that because the lightsource is SO close to the cube and so low to the horizon, that shadow is going to look distorted and wrong, simply because it's out of the field of view. If it hung farther back in space and higher up in the air, the shadow would probably still be distorted, but not nearly as much.
Okay, here is a remake. Ignoring the distortions, I still can't determine the angle or intersection for the middle shadow line. I want to find out the size of the cast shadow using the light source. I don't know what you mean by plotting a line from each vanishing point.
Part of the issue you're having with your example is due to the shadow extending far beyond the cone of vision well into gross distortion. Significantly raising the light source will result in a more reasonable shadow within the COV.