Space needed for time machine back up

I have a 128gb Macbook Air that I would like to back up to Time Machine before switching to a new Mac next week. Currently, 60gb of this 128 is being taken up for the "System." I was wondering whether for a one-time Time Machine backup (I've never done a back-up before), a 128gb USB flash drive would be enough? I've read that you need more space than you have on your machine for your backup disk, but I was wondering whether 128gb would work since I'd only be doing one backup.

Thanks!