Dimensions: 38.8 mm diameter x 7.7 mm H (24.3 mmH expanded)
Works with: any smooth, hard plastic case.
Doesn't work with: silicone, highly textured surfaces, and many soft cases. Does not stick "as well" on iPhones themselves.
While it does stick adequately to a base model without a phone case (and sometimes to Pro/Promax depending on circumstances), it will not stick as well as it does on a suitable case.