I don't remember a way to do this on my old iPAQ (runs PocketPC). I use a Zaurus (runs Linux) now and it can be easily done, however. For a small font change you would just link smaller font size files to bigger ones in the font directory. For more options you would have to download more fonts. If you couldn't find the size you want you'd have to run one of the convert utilities to make them from a true type font/etc or flash one of the kernels that supported true type directly.
So I guess in summary, yes it can be done on a Zaurus. It would be easy for me, impossible for your mom, but likely quite possible for you. Linux offers a wealth of customization options like that. I've read reviews saying the basic PIM functionality and what not on the Zaurus units is easily usable by non techie people, however. So it would probably work out provided you're willing to do the setup for it.