We were able to bring in an average size (like a schoolbag) backpack with no issues whatsoever. Food and drink is fine, but don't have any open containers while going through the security line. Also, half of Court 3 is free, so if you get on grounds in time, you don't necessarily need to buy the tickets for it. In fact, the free half is better as you can also see Court 4 if you go to the top rows (unless they changed the paid/non-paid sitting). But you have to go straight to that court and fast when gates open.
Right, we did just that last year but the luck was not on our side. Zvonareva vs Jankovic - great on paper, but absolute horror show in reality...
Obviously our definitions of average slightly differ. We carried enough food for a Mormon family.