It works best at around 230F to 250 F or so. A double boiler won't get that hot. What makes it work well is that the boxes are literally "fried" in the wax and rosin.
I didn't measure, but I suppose if you knew the volume of beeswax (or paraffin if that's what you are using) per pound then you could do the math. I honestly don't know, but I started with about 100 pounds of wax and 50 pounds of rosin and went from there. In the end I went through 150 pounds of rosin and 300 pounds of beeswax and it's till full and I dipped 500 boxes, 350 bottoms and 100 lids. All eight frame mediums. Plus a few long hives that I dipped first one end and then the other and then painted the space between with the mixture where it wouldn't fit in the tank. Plus 100 2 frame mating nucs or so, and probably fifty five frame medium nucs with lids and bottoms.
If I had it to do over I'd make the tank 24" deep which would hold three medium boxes or two deeps plus one on end in the middle (with eight frame mediums and ten frame deeps or two with ten frame mediums). Then you decide if you want to do full size boxes or eight frame boxes. If eight frame boxes I'd go with 15" by 22". If ten frame boxes, I'd go with 18" by 22". You need a little room around the edges for the bubbling when it's frying.