Darth Satan got it first and deserves the credit, but I couldn't resist doing this in Photoshop:
I took the picture in the question, found a frame from before it where the Executor hasn't hit the station yet so I could see how long the nose is, did a proportional resizing because the resolutions weren't the same, overlaid the full Executor on the crash frame so I could see the full size from stem to stern, measured that in pixels, adjusted it up for the rotatation of the angle of the ship against the pixel grid, to get:
250px Executor = 19km
Then I got a fairly large full Death Star II image and scaled it up proportionally:
DS2 = 160km
250/19 = x/160
x = 160 x 250 / 19 = 2105px
Then I rotated it 90 degrees to get the angle of crash more correct than it would be coming into the top of the Death Star (since during the battle, the main laser was aimed at the fleet battle. Also if it hit the top, it would have landed right next to where Luke, Vader, and the Emperor were, so they would have got quite a view and quite a shake.). Then I touched up the mixing of the images, such as fixing the black line between the explosion and the ship.