For tuna fishing I like my baits to swim, I use very light sea witch heads and make my pin rigs with 1/8 - 1/4 oz chin weights. A skipping balleyhoo also works, just make sure your baits don't spin.
Most popular size for YFT is Medium.
If you want more first hand information, send me a private message or an email to firstname.lastname@example.org and I will respond. Chumlord is having a rigging seminar in his garage this Saturday and if you can make it you will learn a lot first hand. This will be HANDS ON.