You can find the graphic limitations of your specific machine and lens on a glass ball by engraving a series of concentric two-point circles at 0.125-inch increments. Focus at the highest point and center engrave using that highest point as the home position. Check to see how far out and down the ball you are getting acceptable results. That is your graphic size limit.
You may see some distortion as the engraving goes down the side of the object; that should be considered as well when determining what is acceptable or not.
In my case, for my machine, I limit my graphics to a 1 1/4-inch square for a 2.625-inch diameter ball. When using the rotary attachment, I limit the width only of the engraving to the same 1 1/4 inches.
-Jim Puentes, COOLaserCraft