The biggest readily available is a 1250 x 380mm (external) 34kg propane tank, or the slightly shorter one as well.
My brother is pricing a rolled mild steel tube with about a 500mm (20") diameter. Once done we'd weld round sheet to the ends to make our cylinder.
Length doesn't matter once over 2' as a friend is an amazing welder and helping us with the build.
First question is how thick ideally would the cylinder need to be? Cylinder will hopefully be 1000 x 500mm.
Firebox will be square with insulated wall and a thicker steel I'm guessing.
Thanks for any info :)