This ball caster uses a 1″ diameter plastic ball and has an overall height of 1.1″ (29 mm). The two-part housing snaps together to securely enclose the ball and three plastic rollers that reduce the friction between the ball and the ABS housing. The base piece of the housing has holes for three #4 or M3 screws (not included).
This ball caster kit includes a two-part black ABS housing, a 1″ diameter POM (also know as acetal or by the brand name Delrin) plastic ball, and three ABS rollers. When assembled, the ball caster is capable of rolling in any direction with low friction, making it suitable for use as a third contact point for wheeled, differential-drive robots weighing up to around ten pounds.
This dimension diagram is also available as a downloadable PDF (133k pdf).
More technical information at Pololu.