My basket is 12x12x6 inches, it holds what seems like a lot of charcoal to me. I have never cooked it empty before, I generally always fill it up completely before I start cooking. I have done briskets and pork butts low and slow for 12-14 hours and still had over half of a basket left at least. That is about the longest cook I can imagine so I guess my answer would be...
XL Big Green Egg, Weber Performer Platinum, WSM 22.5"