According to the official site
it can be done through the menus. Did you try right-clicking on the background? It also mentions holding shift and pressing +/-.
The manual page
surely says that but I'm wondering if some functions of Xterm are only applicable on other platforms.
In any case, it looks like you should be able to adjust the font size by editting ~/.Xresources
It really seems like a simple question but its more complicated than I thought. I hope this helps.