There are three font sizes with valid uses beyond designers trying to justify their existence: small, medium, and large. Likewise there are three valid fonts:

There are three font sizes with valid uses beyond designers trying to justify their existence: small, medium, and large. Likewise there are three valid fonts: fancy (serif or non-Latin equivalent), plain (sans-serif or non-Latin equivalent), and mono-spaced. This means only 9 font sizes need to be decided. The software can provide reasonable defaults, and a simple UI to allow to user to change them, which won't take long. The only difficulty is when a font doesn't provide full Unicode coverage. Full coverage will have to be assembled from multiple fonts, which means the sizing won't necessarily be consistent. But in this case consistency is already lost, so I see no need to further complicate the UI. The user can still tweak things with fontconfig or equivalent.