They come in small, medium, and large, apparently.
Yeah, but what is weird is that the medium is the widest. How does THAT make any sense?
Well, I have a Sanyo ICC-82D, which is close to the screen-used prop (maybe a little wider), and the inner diameter of my belt holster is 4 inches. The top of my calc board is 4 1/2" and the bottom is narrower, so it drops in there and hangs just nicely, but I have it secured regardless.)
The length of the leather holster should not matter, as your board will be obstructing the view of the bottom anyway.
I think I would go medium. Gives you more leather to work with in case something goes awry.