If the diffuser you're considering replacing is brittle and flakey, then it is recommended that the complete jet internal be replaced.
This degradation is caused from an improper balance of Total Alkalinity, Ph and Calcium hardness. If the diffuser is brittle, then chances are other parts of the jet have been affected, such as the bearing raceway and internal clips
Diameter = 2-1/8"
Length = 3-1/2"
For complete Poly Storm jet internals click on link below
Complete list of Poly Storm jet internals