I would probably use an 8 GB flash drive for x64. The instructions say at least 4 GB free space. The x64 .iso gets really close.
Trouble's link gives a lot of good information, as he has mentioned. Hopefully that will work for you.
Remember, you are trying to make an x64 usb version, but your XP is 32 bit, so if it has to run on XP it needs to be 32 not 64, even though the media produced will install x64..