I do 10 gal batches in mine. https://www.homebrewtalk.com/f11/how-big-your-mash-tun-needs-123585/
Great thread...thank you.
Sorry I missed so much. I had a really thin mash when I filled mine up. I also left out that it was my mash out water that got me to the brim of the cooler. Sorry for that.
Also, to address something that wasn't pointed towards me...When I mashed all that grain I brought most of my grain to temp in a pot over a burner from 130 to 157, which when added to the cooler I only lost about 2 degrees and it didn't lost more than 1/2 a degree over the 90 minute mash (it was really warm that day).
I am trying to keep my steps as simple as possible so I guess I will just play with the temp and see how it works with different amounts.