The ideal approach would be to poll Kodi users about their TV size and the distance they sit from it. This will produce a bell curve, with something like 40" size and 10' distance in the middle and extremes like 13" and a mile on the left, and 120" and 1' on the right. Then you make pains to offer text sizes that are easily readable for the middle third of the curve.
Thing is, we already know what sizes work-- TiVo did a ton of usability studies on this. When creating their SD UI back in 1998, targeting 4:3 CRT sets up to 27" large at 10 feet distance and 720x480p resolution, they used ENORMOUS fonts, with the entire screen functioning as a single horizontal list.
(Note, this is a native 4:3 image stretched to 16:9.)
I personally believe the original TiVo UI is the best 10' UI ever created. It is
dead simple, with easily understood navigation-- left backs out, right drills in, and it's super easy to read yet not busy, retaining tons of whitespace. It has not yet been surpassed, including by TiVo themselves. Which takes us to...
The HD TiVo UI. They built their HD UI in 2010, targeting 16:9 1080p sets up to 42" large at 10 foot distance. They made more efficient use of the extra horizontal space and display sizes and moved to smaller fonts.
When you compare the font sizes on the HD UI TiVo to the official Kodi skin, the readability gap is stark and immediate.
The text size difference is obvious, but there's more to it-- the DVD covers for movies are attractive, but could anyone without Chuck Yeager's eyes read the Christmas Carol cover 10 feet away from a 40" TV? It's low contrast, tiny text. And the movie on the top right, with the green portal thing-- there's no title at all in that graphic! This is a case of cosmetics trumping usability, the exact opposite of my beloved SD TiVo UI at the top of this post.