In 2000, the diameter of ITTF approved table tennis balls was increased to 40mm from 38mm.
Before 2014, balls were usually made from celluloid, although other similar plastics material are allowed (but not popular among players).
Since 2014, balls are usually made from a plastic material. The minimum diameter of every ball must be at least 40.00mm and its maximum diameter must not exceed 40.60mm. (Taken from the ITTF Technical Leaflet T3: The Ball)
The weight of the ball must be 2.7g, and the ball for ITTF competitions must be white or orange, and have matt, not shiny, finish.
Needless to say (but I’ll say it anyway), the ball should be spherical!