Soil conditions, speed of travel, depth are all big factors.
I have 27
PTO and run a 72" tiller in heavy clay, last summer (read dry hard packed clay) I broke ground with the tines of the box blade digging in several inches. Then on with the tiller and I tilled about 4 inches or so down, then I lowered the tiller all of the way down and my little Kubota never missed a beat. This spring I retilled the garden and it was no problem.
I would go plently wide you can always remove some tines, or make a few passes if needed.
This spring before the clay dried out I tilled some virgin ground and had no problems going down 6-7 inches.
steve