I didn't find a way to grow the boot partition on a live system (I think OS X has been able to do that for some time now?). I used BootIt, free for this kind of partition work. Quick download, quick self-install on a USB stick, reboot, unnervingly unresponsive while working, but finally problem-free.
In my case I was growing the boot partition into free space. And this XP uses some Intel SATA AHCI driver, and installed in a non-standard way at that, which made me a bit weary. But everything turned out right.
The next best option seemed to be GParted Live, which also has an easy way to be installed on a USB stick via TuxBoot.