Around here we don't figure the dumpster size on weight, we have to figure it on volume, because they all only allow you to fill it to a certain line in the bottom, "like 18 inches off the bottom" for concrete. So we have to figure out like a 10'x10'x4" slab if it would fit into the allowed fill line of a 12 yard dumpster or an 18 yard dumpster etc... Generally the dumpster companies I use will tell you what you need if you tell them the size of the slab you are removing.

Also, I don't know if you have this in your area, but around here we can get a good discount on the dumpster if we use a concrete recycler's dumpster instead of a normal dumpster company. Of course they get really picky about how much rebar and how 'clean' they want the stuff.