Not all PCs will boot from a USB key.
And for those that will it has to be in a specific format. Your BIOS may have options for USB floppy, USB hard disk, USB zip drive etc. Try all those options to see which if any will work.
Unsure why you are getting that size. You may wish to try and delete the partition involved and try again.
Other methods of doing what you want can be found here
http://www.bootdisk.com/pendrive.htm