I fire with Propane in a medium size downdraft. My kiln is a 36cuft downdraft with 4 75,000 BTU venture burners. I use two 250gallon tanks in tandem to fire and I have never had a problem. Rather than measure everything I talked to my propane provider and the dual 250's were best because of local regulations about tanks over 250. As it turns out I get about 5 firings before the tanks are at 30% when I call to get them refilled. I found out the hard way that if I try and fire with less than 30% the vaporization cooling is too intense and freezes too quickly.
Prior to this I had a small 8.5cuft Torchbearer type kiln with 4 venture burners. I used two 100lb tanks in tandem. (I do not remember the gallons, but they were the tall skinny tanks). The consumption of propane was fine with those however I would always freeze the tanks up on the last firing, I think it was the 4th. Which I got into trouble a few times having to pour water over the tanks to warm them up. I hated firing that kiln and so glad it is gone.