The SF Giants allow bags that are soft-sided and smaller than 16x16x8 inches, which is roughly the size of a small backpack. You can also bring clear bags made of plastic, vinyl, or PVC, which is basically a fancy way of saying " Ziploc bag on steroids". Just don't try to sneak in a large duffel bag, or you'll be sent packing (pun intended)!
Now, you might be wondering what's not allowed, and the answer is pretty much anything that's not soft or clear. Think hard-sided bags, like coolers or briefcases, which are about as welcome as a Dodgers fan at a Giants game. And please, for the love of all things baseball, don't even think about bringing a large purse or a bag with metal frames, or you'll be denied entry faster than a pitcher can throw a fastball!