Personally, I believe that 20" will be way too big. I've split the difference, between 14-15" diameter. I've heard it said that'll burn for upwards of 15 hours...this thread,
http://www.thehotpepper.com/showthread.php?p=224276#post224276 has a pic at the end of a very nice firebox. Many people base it on the charcoal grate of one of the Weber kettles. I'm gonna use Ex Metal cus I'm a po boy.
Maybe not quite enough starter coals?
I had my thermo in my offset smoker today, and it was 150. I wasn't cooking, it was just 150 in there. Next week, I'll light 2 briquettes, see how warm it gets...
dk
P.S. Watch the banners there. One of the sponsors is a familiar name....