More of a half gallon sized fridge. I think you could put in a gallon but you'd have to take out the shelf and sit the milk on the floor of the fridge. However you could probably fit 2 half gallons in the door or on the floor of the fridge better. If the price is close I'd do two halfs.
I tended to buy a quart (used two over the week) and I put it in the door with plenty of room to spare.