Here are some ideas [which may have been discussed before] that could help free up space with the furnace.
1. When inserting items in the furnace, they automatically stack with your other items. So placing 10 iron ores individually into the furnace will cause all your iron ores to auto-stack into a stack of 10. Then when they melt they stack into a stack of melted iron. Really, there is no reason not to stack your items when putting them in the furnace. A stack of 10 iron ores when melted gives the same practice as ten individual iron ores. Though, I don't think iron ingots will melt properly when stacked in the furnace right now.
2. Have a seperate piece of equipment for combining ores. So you take your iron ore and coal, place them in the 'combiner' and get a stack of 10 steel sludge [or coal sludge depending on the recipe] then put that stack in the furnace to melt into steel. This would save some time since you could quickly combine your ores and then just use 1 slot in the furnace to melt it into steel.
3. If someone places garbage like animal parts or potions or anything like that in the furnace, it will burst into flames or explode causing injury to the person who put it in. Nothing fancy, just turn the item to dust and have the furnace cast a lvl 1 flame burst on them. If its a potion then it's a lvl 10. A stack will cause it to cast several flame bursts and if the target leaves it stops casting and turns all non-metals into dust. I know this could cause all sort of dangeous bugs... but it would be a pretty unique feature. Also pretty somewhat realistic (Seriously, playing with fire is dangerous).
I know this has been discussed before and these may not be the best solutions [I admit #3 would have potential to go really wrong] but anyway, any better solutions?