I have used various sized blocks. 8 inches was the largest, but only because I was using old jeans for the back and that was the best size for getting the most usable fabric from the jeans.
My personal preference is to not use batting. With denim it is heavy enough with one layer of denim and one of flannel. With all flannel, I like to use a coordinating solid flannel in the middle. You get fluffier edges and you do not have the X the centre of each block.
I just realized I did do one 10 or maybe 12 inch block one. It was someone else's UFO. Every other block was a 4 patch.