I have 18, 28, and 45 mm cutters. I almost always use my 45 mm one. I read somewhere that the 28mm one works best in the shape cutter so I started using that one for the cutter and actually like the cutter now! Although I see that some prefer the 45 or the 60 for the shape cutter.
I have used the 60 at a volunteer event and it worked fine. So it's just a matter of personal preference. I don't usually cut more than 2 layers at a time.
Connie