One of these small things that comes up occasionally and that you can never remember at the time but always have to reinvent or rediscover.
Occasionally, a USB stick will end up with an odd partition layout / an Apple media ID or something that means that the size isn't reported correctly or means that you can't use it.
Plug in the USB stick :
the last line you see is the line showing the USB stick you've just plugged in - say /dev/sdb
From a command line as root:
dd if=/dev/zero of=/dev/sdb bs=512k count=4
Writes 2MB worth of zeroes to the front of the USB stick, overwriting entries already there.
After that, parted /dev/sdb should enable you to partition the USB stick or you can use dd to put a bootable .iso file on it.