Fuel is used very slowly compared to ore. One coal can smelt many units of ore, even in a stone furnace.
Not sure of the exact ratio, but quick math suggests around 30 ore per. Given a cargo wagon can hold 40 slots of stuff, you only really need 2 slots for fuel. Less over more wagons. Except that the iron plates stack twice as large, so that throws it off more, so Ima guess 5 slots for fuel, because don't feel like mathing for something I don't plan to use.