16 or 32GB is great. Don't really even need USB3 speed but it doesn't hurt. I got a 16+ track recording from an Allen & Heath QU series on a USB2 stick.
Make sure it's got mostly empty space, but it's handy to keep some media files on a drive for testing device playback.
I keep some .MOV, .MP4, .M4a, .WAV, .FLAC, and a few .JPGs so I can test out media players and smart TV's at work.
Thanks guys. I'm guessing 16gb-32gb and making sure that it is USB 3.0 compatible (for speed) is a great starting point. I will make sure to put a few in my bag and leave a phone number on the drive itself just in case.
I make a folder on each thumb drive called IF FOUND PLEASE RETURN ME TO __ (my info)