The "Mayor's Formula" would estimate 5 tons for every 2000 sf. That would be 13 tons. If the basement/garage area isn't included in the 5200 sf, you might need extra for the heat being put into those spaces. If you're also making DHW, that takes a lot of btu's. Better that it's coal btu's than btu's from more expensive fuels, but DHW still uses coal beyond that needed for space heating. Overall, 20 tons still seems like a lot, but probably is not over what it should be by as much as you might think.
Definitely make sure the draft is not excessive, and check the ash ring to make sure you have a good fuel-air mixture. After a long run there should be 1-2" of ash around the outside - no burning coals going over the side, but no wide swath of ash, either. Make sure the gaskets and fit are good on the ash and fire doors, and that the surfaces of the combustion chamber and heat exchange areas don't have excessive fly ash buildup.