If you want to go as big as possible you might be able to get a feel for size via a fly cutter. Your question can't be answered really as size, depth of cut, speed and feed rate all determine the HP needed. More cutting points helps a bit with feed rates.
Personally if I had your machine I would probably be more interested in horizontal milling if I had the full horizontal attachment that supports a bar for cutters at both ends. Flat surfaces rather than dished due to head alignment errors.
You might find some demo's that use the machine on youtube.
John
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Edited By John W1 on 09/09/2015 13:32:42