What are you calling a 'terminal emulator'? I think most support multiple fonts at different sizes. Apple's Terminal does, as do all the terminal emulators on my Centos systems.
In tmux, you can have multiple panes inside of the same terminal window. Apple's Terminal lets you change the font, but as far as I know, there isn't support for multiple font sizes inside of the same terminal window.
Really? I thought most terminal emulator only let you use one font at a time. If the one I use let's you have text in multiple fonts displayed at once, I certainly don't know how to do it. Or by "multiple fonts" did you just mean that you can change the font on the entire window (and that you have multiple options for doing so)?
rxvt-unicode allows you to have multiple fonts at the same time for falling back when a character is not present in the default font. For example my terminal uses a different korean font for hangul and a specific japanese font for kana and kanji in addition to my default font for european script.
