Its actually very easy, and time consuming, to figure out exactly what the forces are on that system in the event of a collision. The point is that unless you have a plate underneath what is shown, that the size of the plate doesn't really matter, the force would travel along the plate and down to the sheet metal, at which point the shear of the sheet metal becomes the determining factor of the strength of that particular joint.
here are some examples of NASA / FIA approved methods of what you're building:
NASA requires at least a 3 x 4 x .125 footprint that mounts directly to the body