Got it fitted, thought I'd add my experience:
Not much bigger on paper but it sure feels it in the car. I love the shape but could do without the the chunky plastic surround. For the money it's a nice mod though and ties in with my oval wing mirrors. Just feels more roadster, like it should.
I also like the twist feature for the dipping rather than the 'flick' tab style on the original.
The pic below shows the triangular mounting that is at the end of the arm- the arms are different but the mount is exactly the same just twist the original off and twist the new on one - don't pull or push or whack it, just twist!
As mentioned above do not remove the Nirro from the arm via the ball making as that just makes the hole bigger and your mirror gets floppy or you can easily break the plastic inside the ball joint.
To get at the wiring the mini mirror glass pops off easily - just in lip the surrounding plastic and it splits in two so you can take out the wires if you don't need them or change over if you have wires in your original.
When you buy one just make sure that the plastic cover for the arm is still present, it want on mine is mine has an industrial/lightweight look about it (!)
Finally the seller I bought from was very good - the mirror arrived with a big scratch on it, contacted him via eBay and he arranged a courier to bring a replacement and collect the old one the next day.
Lucky I opened the box as the courier waited as the replacement was for a hardtop mini, not a convertible so has a different arm. No problem, just swapped the glass over whilst the courier waited.