According to the description the head is 25.5 mm. Lens should be 22-23 mm. One user says there is no off function. I don't know if that is true or just an issue with his light since no one else mentions this.
This light definitely has an off mode. The person claiming otherwise probably has a defective light.
This light is nice (solid body/threads, zoomable, tailstands, fits protected cells, programmable, etc.). For a budget light it is surprisingly well-built, with smooth zooming. The beam is a little ringy, but not too bad.
That said, I rarely use it myself, since I can get better performance (higher high, lower low) from a much smaller (but 4X more expensive) 4Sevens MiniX 1xCR123A light. The MiniX is really a different breed though, since it doesn't fit protected cells and has no zoom or programming. I found I get plenty of throw and spill from the MiniX on Med/High with a RCR123A (not officially sanctioned), and I like the MiniX mode spacing and hidden flashing modes, so I never need the zooming or programming features.