You're not ordering a generic box when the job is Construction Dumpster Rental: it is debris that arrives in bursts tied to which trade is on site that week. Ask about this specifically, because it rarely comes up unless you raise it: clean fill must be separated from mixed debris or the whole load is charged at the mixed rate.
Two ceilings apply to every load: how much fits, and how much is allowed to weigh. Dirt and concrete hit the second one first; furniture and boxes usually hit the first. This request comes from builders and general contractors more than any other caller, and the volume they generate is the reason 30 or 40 yard containers are the ones kept in ready supply for it. Have an answer ready for two things: how long the container sits on the property, and whether the load includes anything that isn't allowed in it.
This load blends heavy scraps with lighter debris. The heavy pieces set the ceiling, so the container fills by weight before it fills by eye. Get the swap-out cost in writing if the project runs past the rental window -- a second container costs less mid-job than a rush order arranged after the fact. The size on the container door is a legal limit, not a suggestion. Treat the fill line as the actual boundary when you decide what goes in first.