I made those modifications to print.cpp and print.h, and it worked just great on my DUE. I am now able to display the true size of my 16 GB SD Card:
Volume size (bytes): 15923150848
I didn't change the methods to size_t though, it worked fine without that change. (I did try it, and it threw a bunch of errors, so I left them as void).
As you noted, the change does add about 8KB, but heck, I've got a DUE with 512K program memory, so I can spare the extra 8K. Woot.