what is the optimal usb stick for privacy? I dont think thats a question with an answer a usb stick is a usb stick.
In terms of size 4gb would be the minimum but if you want to have extra space to use for persistence storage you might want to go with an 8gb sized stick.
I would suggest a usb3 stick if you have a usb3 port on the machine for the increased speeds, but a usb2 stick will work too.
If you use an sd card id recommend a class10 sd card for the increased speed but with usb2 or 3 the speed is pretty much one in the same. There are cheaper usb sticks with slower read/write speeds so you can spend a bit more to get a slightly faster drive with better read/write speeds, see the sandisk ultra series.