Figuring on about 11' high, or so for a hopper, I'd start at about 14' high for the front. Use that for the starting point and estimate the length from the second picture or just make it to fit the space available or whatever is appealing to your eyes.
I would hazard a guess to say that a bit of "poetic license" was used in modeling that scene. No way would they dump a hopper on the tracks, then try to shovel the coal out from under it and into a nearby building! Just not practical, even given Depression Era wages!
Too easy, if you can't elevate the track just lower the scenery!
No really, create a little low land to break up the prairie and viola!
Why not put the bin below the track with an elevator running from it to an opening in the roof of the trackside building? That way there wouldn't be any need for an elevated section of track and the extra space needed for the ramp.
We had a fertilizer dealer where I lived that had a ramp leading down to a pit beneath the siding track where they would spot a hopper loaded with lime. The car was dumped into the pit and an endloader was used to run down the ramp and get a bucketful of lime and backup the ramp to dump it in piles on the ground. Didn't take long to do a whole hopperfull.
Too easy, if you can't elevate the track just lower the scenery!
No really, create a little low land to break up the prairie and viola!
Actually a lot of these building had coal delivered in gondolas. Coal was generally moved in gons from the 1880's to about 1910 in the east and in gons until after WW2 in the west. If you look at western railroads in the 1940's and 1950's, hopper cars are pretty rare compared to GS gons.
I got one of these conveyors for one of my projects
http://www.walthers.com/exec/productinfo/933-3520