I used the default size, and it was bigger than I thought, but that turned out really well I think. I like the larger size. About the size of my hand.
This thing rolls and rolls. I thought the hardwood floor in my house was level, but this thing picks up speed when it goes in one direction, so the floor must not be level. Very cool.
I made it with higher infill than I usually use - I think it was 30% or 40%, which gives it some heft, which I think makes it work better, and feel good.
The friction fit of the plugs and two sides is perfect, no glue required (yet at least).
Video of it rolling along the floor: https://www.youtube.com/watch?v=lWDm52TECGo