For simplicity's sake just make it 4x4x8, that will hold approx 3 tons of coal. A hinged lid or light removable roof makes for easy deliveries however that's not necessary for access to the coal.
(This part is hard to put into words) On one of your 4x4 sides instead of a solid wall leave it open. Place 2x4's or really any width board on both sides of the opening perpendicular to the ground, so you have lip. Cut boards that you can place behind the lip, during delivery you put the boards in as the coal is coming in, the pressure of the coal will hold them against the lip. To visualize this it will be something like a guillotine, the lips being the sides and the slats being the chopper.
Leave a small space at the bottom and shovel the coal from there and remove the top slats as you remove the coal. Eventually you'll be able to shovel from the top and once all the slats are removed you can walk right in. 8)
If you check the coal bin picks you'll find a few examples...
FYI, GENERALLY USING ALL CAPS IN A FORUM IS CONSIDERED SHOUTING AT SOMEONE....