USB Stick Time Machine Setup Reversal?

Hi,

I attempted to setup my 64Gb USB stick for Time Machine on Macbook Pro but annoyingly it didn't work as it said it required just over 100Gb.

For some reason the stick is now only showing 200mb capacity when it should be 64gb like it was before I allowed the Macbook to format it in preparation for Time Machine backup.

How do I revert back to it's original 64gb capacity? I've tried to do it on a work PC by formatting it but it doesn't work. Formatting just resets to the 200mb capacity, I can't change it.

Any help please?