I have used this two years in a row.  This bag is acceptable for NFL events and music festivals.  We fit a ton of stuff in this bag, but we were strategic with placement.  It will fit in a standard locker, but you will have to carefully push it into the locker.
We had a plastic/canvas blanket folded up.  It takes up about the same room as a t-shirt.  Sunscreen, cell phone chargers, power pack, socks, a light up mask, a clutch wallet, hoodie, and a bunch of other stuff.  I cant remember everything.
**Tip** Place small items in baggies or other smaller zip up pouches.  This will make it easier and faster to locate the items you need.  And, if security makes you empty the bag during the security pat down, it will save you some time.