How big should a diaper bag be, and what should go in it?

I had two bags, for just the reason you mentioned. The bigger one held an extra set of clothes and extra food, they both had the baby basics: diapers, wipes, couple toys. It was really easy just to grab and go with the small one for trips to the park or just the store. The bigger one I used for day trips, or if I was planning on visiting friends for a few hours. Good Luck!