Google says energy of combustion for hard coal is 25 MJ/Kg. E = mc^2 gives 278 picograms of mass lost per kilogram burned in ideal combustion. Consider the amount of earth's mass that is coal and reduce it again by 278e-12 / 1000 to find Earth's mass loss if you did complete combustion of all coal on earth.

Fun napkin math but probably wouldn't make my list of top 10 concerns for humanity.