Fortunately, I happened to have HD800s with me now.
I measured the size of the outer grill of the HD800s's 56mm driver. From the outer edge of the mesh to the shoulder of the other side of the mesh was 50mm.
We can see Yamaha's 50mm diaphragm and its cover metal part in the photo of the headphone festival in Tokyo this September.
Then we can compare whole pictures of those two on the same scale.
Measuring from the first image of the comparison, the diameter of the earpad of YH-5000SE is 28% shorter than the height of the earpad of HD800S.
Photos
YH-5000SE:
Yamaha's Official web
HD800s:
Sennheiser's Official web +
a Webshop (side view)