It's about the length of a disposable lighter, with everything in proportion with a normal mouse but SMALL. It's kind of cute.
The retractable wire is the only minor gripe I have with it - it has a small plastic spindle that the cable wraps around, much in the same way that a garden hose wraps around it's housing, except it seems to be spring-powered; the problem is that if the wire gets even a little bit out of alignment, it jams on the spindle and doesn't retract. Which just means you have to take a couple of seconds to ensure that it spools in s-l-o-w-l-y and carefully... I feel slightly mean docking it a star for this, but it's not perfect so NO 5 STARS.
Even Mickey would trade his missus in for this svelt little beauty :)
UPDATE 30/10/08: The mouse has developed a loose connection in the wire, resulting in the computer recognising it, then not recognising it ad infinitum. It's not really worth using any more - it's the constant extending and retracting of the cable that does it; I suggest you treat it with great care and gentleness, as the cable ends don't protect the connections very well. It might be better to go for a slightly more expensive one with a longer life.