I don't drive a Nissan.
The format for usb stick is probably Fat32, your computer should be able to format a usb stick automatically.
As for size of drive, you can go any size but the larger size maybe a problem for your deck to load all the files. For a car 32 gigs should be plenty and being mp3 that should be more than enough, don't you think.
You could also probably use wav files if you want better quality. An 8 gig usb stick could probably hold about 10 albums or about 150 songs give or take.
As for Files you usually put them in a folder, label the folder by album name or genre like pop, rock, etc.. Your deck should show/list the folders and the you just click on one, select a song and it should play. You may also be able to select random, shuffle, repeat, sequential play.