I have never used a pusher .......... but unlike a rear blade that rolls the snow away, so the load is semi constant, a pushers load gets to the point where the weight will over run the grip.
The type of snow, the amount of snow changes the load obviously - you have chains on tractor? If grip is a going to be a concern, I would use the 60".
Another thought - if you buy the 66 and then find you cannot hold grip, you will have to work inside those limits every time you push snow - if you get the 60, you will make a few more passes to cover the same distance.
Driveways are typically 10' wide depending - I will say the 60" will do the job just fine. What is the cost difference between the two models?