it has to be stock physical size, but can be upgraded internally. Mine is stock.
When you say stock SIZE, you mean it can be like, 200a H/O but has to be direct fit?
some of the rules, car has to look stock, 2 runs of wire front to back, 1400 cubic inches of total battery(2 total), 240 sq inches of cone area max, stock size alt. it's a tough class.