For that size chamber, your numbers are as follows.... firebox: 9328 cu in to make 110%, exaust: 466 cu in, minimum total intake: 30 sqin, throat of 74.62 sq in.
An octagon volume is measured by the width of its sides and the legnth. I’m not sure what you had in mind with “diameter of 21 inches.” With that said, a true octagon with 8 equal sides 9.4 inches wide and a legnth of 22 inches will give you the required volume.
Hope this helps...and welcome to the forum.