None of the standard pan conversion charts that I could find had that size, but it looks like your recipe will work with an 11 cup volume pan. If this means your brownies are thicker or thinner than the original recipe, then adjust the cooking time accordingly. For thicker brownies you may also want to lower the temperature to keep the top from burning while you do the extra cooking.
This happens to mine, and they are most definitely stainless, not aluminum. Our stainless sink gets the same way. I assume it's from all the minerals in our water. I have found that mixing up some Oxyclean and water and a couple of minutes' soak and a scrub with a plastic scrubber will get rid of it (both from the pan and from the sink). It comes right back, though.
I haven't found that it has any negative impact on the usability of the pans, and doesn't shorten the life significantly if the 14+ year old chef's pan I use almost daily is any indication.
I did a little more research, and it's definitely mineral haze. Recommended treatments are Barkeeper's Friend (mild oxalic acid) soak/scrub, or boiling some vinegar, either one followed by a thorough rinse. In theory, if the haze is left alone it could develop into buildup, which would begin to degrade performance, but in practice it's not an urgent worry.
So if it bugs you, clean it off with mild acid or oxygen bleach. But expect it to come right back. If you only removed it once a year, you'd be a couple of years ahead of my cleaning schedule, and I haven't seen any harm from it.
Best Answer
You can use about any pan that you could cook rice in. I've made Paella in a stock pot, hotel pan, and a Dutch oven. Honestly, the paella in the Dutch oven turned out better than it usually does in a paella pan. The heavy bottom of the dutch oven allowed for a much better layer of toasted rice at the bottom. For very large quantities a hotel pan is most convenient, but you may have better luck cooking the rice separately and tossing it together with the other ingredients at the end.