Just an FYI, but I was curious so did a bit of quick (very quick) looking. Both THX and SMPTE refer to horizontal screen width when discussing screen size, whether in X.X times the width or a horizontal viewing angle. However, this is for a certain aspect ratio (couldn't quickly tell if it was 1.85:1 or 2.35:1 they were referencing), and all of the more recent discussion of this topic I have seen relate screen size/viewing distance considerations more to the screen height than either diagonal or width. I suppose that's because we have a much wider comfortable horizontal FOV than vertical (even though our total FOV is much closer to 16:9 or even 4:3, which is I think why this was the original aspect ratio, research over more recent decades has shown that we may have decent vertical FOV but it isn't comfortable to shift our eyes up and down, while horizontal scanning is quite natural and well within our comfort zone).
I wish I could find a handy reference link... I know I've seen them in months past but can't locate one quickly now searching through the CIH forum. I think the current recommendations are to sit (or size the screen) somewhere between 2x and 3x screen heights to the primary viewing position, regardless of aspect ratio.
I just don't think I've seen the diagonal used to make a recommendation, at least not anytime recently. I'm sure it's out there though. With the changing aspect ratios, using a fixed height does seem to make a lot of sense, and aligns better with the way we view the world.