Is it possible?
If yes, how biG a drive should I use?
Advisable : NO
USB sticks are definitely not as reliable as (portable) hard disks (being it SSD or HDD).
You can find 1TB harddisks from WD or Toshiba under 50$.
Do yourself a big favour and make that extra effort. You will be happy if you ever need to restore a backup. And one day, you will.
I backup to both my music spinning disk and to a 64GB USB stick. I’ve used it a number of times to restore my db without issue.
Do I trust it, yes but only as much as a USB stick should be trusted. Have more than one backup. 1 is none, 2 is one…