If you take a Hobart mixer as being representative of what professional pizza operators use, then I would say that you would need a mixer with a bowl capacity (in water) of 60-80 quarts. Hobart recommends its mixers based on dough batch size. Dough batch size varies in accordance with the type of pizza dough to be made and the amount of water used. For example, for a 40% AR (absorption rate, or hydration percent), the recommended maximum dough batch size is typically 40 pounds (this would be for a thin-crust pizza as an example); for a 50% AR (e.g., for a medium-thick crust), it would be 70 pounds; for a 60% AR (e.g., for a thick-crust pizza), it would be 90 pounds. If you use a high-gluten flour, the recommended maximum dough batch size is reduced by 10%. So, as you can see, the lower the AR and the higher the protein content, the lower the recommended maximum dough batch size.
In your example using 50 pounds of flour and a hydration percent of 65%, that would represent a dough batch size of around 84 pounds (including flour, water, yeast, salt, and oil), or about 76 pounds if a high-gluten flour is used. You would have to use these numbers to determine which of the Hobart models can best handle these dough batch sizes. If you are interested, you can find the answers at http://www.hobartcorp.com/hobartg5/pr/products.nsf/pages/food-prep_mixer?opendocument
. Look at the "Mixer Capacity Chart--All Models". You will see small- and large-capacity mixers, but I think you will find that the answer is 60-80 quarts for your example.