I have a 12 lb turkey in one of those right now... just barely zips shutThe big Ziplocs are 2 gallons. If you're lucky, you'll find them at Wal-Mart sometimes. I'll snag them there when I can, otherwise I order a couple boxes from Amazon when I'm running low and ordering something else. They come in really handy! I use them when I'm resting carcasses in the fridge, I can get two small cockerels in one, or a bigger bird alone. The bigger birds don't fit into a 1 gallon zip bag well at all.
When learning about brining, I was shown a 3 gallon crock. Ceramic, wide mouth jar. Think moonshine bottle without the small top. These were used for pickling meats in brine. The ceramic is acid and salt resistant. Cast iron or other dutch ovens wouldn't hold up. The ceramic coated dutch ovens might work.
The meat goes into this and a plate is inverted on top and holds the meat below the surface. Whole thing goes into the fridge to hold temperature. Since I don't have a 3 gallon crock, I've used plastic (2 gallon) bowls with a plate over the meat holding it below the brine surface. Works OK, but the bowl is a lot shallower than the 3 gallon crock. Be sure to get all the air out from under the plate. Submerge the plate, then work out all the air.
Large ziploc bags sound easier. Mine usually leak. Sharp bone or caught on whatever pokes a hole. Do you put the ziploc in a bowl?
my uncle brined meat in one of those crocks.. it was his wine making crock and when he used it for wine again, the wine soured.. no amount of cleaning made it useable for wine again