Anything less than a big bag is unacceptable imo, but that's coming from someone who operates on the mindset of "you never know what might happen".
I have my wallet, my keys, my phone, my headphones, a snack + a water bottle, my charger, my journals + pens and my tablet.
I also have this bag that has, including but not limited to: lotion, perfume, lip products, a nail file, a nail clip, a mini mirror, pain-relief pills, extra pad supply, bandaids, a mini-tube of adhesive liquid, gum, my toothbrush and paste, wipes, antibacterial soap (highly needed if you get your hands dirty in labs or if you're germophobic)....You genuinely never know when you need any of this stuff.