How to choose the "right" font size for a given page

I asked a similar question in TeX.SE and I am not 100% happy with the answers. I find it strange that there's no standard formula that yields the "right" font size given the page's size. Therefore, I'm going one level above --- leaving the practical TeX world --- and try to find a general answer. I know that it is desired to have 65 +/- 15 characters per line. Is the only way to achieve this is to iteratively improve the document?

I hope that my question makes sense...