I have found Mac USB boot tutorials and have made a bootable USB for PC using Ubuntu. Just looking for the right direction.

Q: I want to know how I can use my USB to boot Ubuntu on both
Mac and PC.

Also I've read about limitations of space Ubuntu can use on the USB for saving and accessing files.
If that's still valid-

Q: What can I do to use the USB's full storage potential.

Hardware: 32GB Transcend USB flash drive, 2007 Macbook, Windows 7 PC