Carried a gallon of milk for the first time in my trek interchange's earlier.
It fit with room to spare. 
I was prepared to carry it in backpack. Instead, i folded up backpack & put in with reusable grocery bag in the outer pocket. At my next stop, used backpack to carry other items (3x 1 liter purell bottles) then finally put a half gallon of chocolate milk in backpack at second grocery store, before heading home.
I regretted having the pannier on the right side, as bike kept wanting to fall away from kickstand. lesson learned, i will carry both from now on. It's no biggie, as the side straps can cinch it to be very flat. Only the rain cover in its pocket really sticks out.
- Andy