Coal bin opening

Could somone tell me how big the opening has to be do have coal dumped into a bin. I have a coal bin uder my porch and there is only a small opening that I'm not sure coal will pour into easily.

If you get it delivered from someone that does it for a living it shouldn't be an issue at all. I even have chutes for going down pipes that I can put into the pipe to maximize the speed of the coal. A lot of people used terracotta pipes at one point and they usually never have enough pitch on them. Same with chutes that go under porches. All kinds o tricks up my sleeve.