I have a 64GB SD XC card for my Canon that my Windows XP computer will not recognize (but a Windows 7 machine will).
Assuming that the card reader recognizes it, what is the maximum size that I can get that my Windows XP machine will still read?
Cards larger than 4GB, and SDXC, are formatted in the exFAT file system. Most newer PCs and Windows 7 can read this system, but your XP system does not. However, you should at least try the Microsoft exFAT driver update to see if your system can support it.
I would also suggest that you only format (and erase) the card on the camera, not on your computer.
There is no maximum size. The issue you are having is that the SDXC card is format as exFAT. This happens to be the default when SDXC cards are shipped and also what cameras format the SDXC cards as.
There are two workaround: