I surrounded a Weber Kettle charcoal grate with 10" high #9 expanded metal, the grate is suspended 2 1/2" above the bottom, therefore there is about 7 1/2 inches of ring x 18 1/2" diameter, the 2 1/2" provides the leg part of the charcoal grate and allows for good air flow underneath the fire.
This gives me about 2 - 2 1/2" all the way round for clearance between fire ring and the side walls of the drum. I've cooked some long 18 hour + cooks and the charcoal did not fill the ring past the half way or two thirds mark, on the next on I'm going to build it with an 8" piece of #9 expanded metal, and I think it will burn at least as long as the 10" filled the way I normally do.
"There is no such thing as a little garlic." A.Baer