The size of a barn.
Anybody that's been doing this for a while is going to be prepared for many different circumstances, I even carry a smaller chute that is specifically for terra cotta pipes.
If for example you're going to have the opening on a side of building a single cider block is sufficient however the more room you provide the easier it is for the person delivering it. The cinder block for eaxmple obstructs your view inside the bin, also you can't manever the chute from right to left.
You mentioned a doorway and as I said these are usually pretty tough unless I'm misunderstanding what you mean by doorway. The bin is going to be to the left or right side inside the door so you have narrower margin for give and take because you have to chute it through the door at an angle, plus the height of the door limits the downward angle of the chute which also limits the height of the bin.
From the above example for me it would be much better if there was simply a window or even a cinderblock directly on the side of the bin. Also not you would be able to build your bin higher, anybody with a truck like mine would be able to put coal into window a cinderblock about 5 1/2 feet off the ground, much higher if you could back up to it directly.
Hypothetically speaking If you had a bin with a window like that and all the space in the world on the other side I could probably put about 10- 12 ton in and never touch a shovel.