We are working with batches of dough that fill a 20 litre bowl. About 5.3 US gallons I think.
That is as much as we can handle: mix up, knead. It gets very stiff, pretty hard work.
So we'd like a machine but commercial machines this size cost thousands of dollars, it seems: http://www.foodequipment.com.au/downloads/pricelist/W_201205_R-PRICE_FED_AUNZ_v11mR3a_A4.pdf