To my understanding:

Many terminals are capable of displaying multiple fonts at the same time, say latin unicode characters in font foo and japanese unicode characters in font baz. In urxvt at least, it is also possible to have one font in a certain size and the next font in another size. However, no font can have a size bigger than the base size, the size of a terminal cell.

Why is it not possible to have multiple terminal cell sizes? For exampleso one line has terminal size 8 and the next line has terminal size 12.