We have here a 2x6x1″ Dan’s Translucent Arkansas Stone, and one of its 2×6″ surfaces has been shaped to become a 25’Ø primary axis ellipse, with a 6.5’Ø secondary axis. 

I do not believe there exists a superior whetstone for finishing straight razors than the convexed hard/black/translucent Arkansas stone, entirely dependent upon its surface preparation, shape, and homogeneity.    

To prepare this shape, you must endure a miserable fight with abrasive sheets and the stone.  That's part of what you're buying here, the other half being the raw product from Dan's Whetstone, which is renowned for flatness (that'll be relevant on the 'other' 2x6" facet, obviously) and density/appearance uniformity.  

For a bevel-setting stone, you're best served by a much coarser abrasive paired with a much shorter primary diameter shape.  But for the final step, I personally do want the 25'x6.5' shape at this point of my sharpening journey, it is too good.   That's not to say that those ideal diameters won't reduce in my life, but they certainly won't enlarge.