There ain't one. 55 gallon will work just fine. Build it with a couple or three shelves, and play with it a little. If the shelves work, then use them. If they don't, then you just have some added flexibility. Its a lot easer to not use the shelves, than to add them later. The large BDS is a 55 gallon drum. Your design is sound.
Nauti-Que BBQ Team
Pitmaster and KCBS CBJ & CTC