Yes, matchbox size, but quite expensive I'm afraid, circa £60 (with own GPS module, which makes it a bit bigger) or £100 (with dedicated GPS module) but all with no enclosure.. but only be circa w20xd20xh45mm
I am in two minds whether to take it any further, though it would provide a convenient fit and forget GPS logger as long as you remember to ensure it's charged up (which it could to automatically off the dyno) and only be limited if you couldn't get a GPS signal or ran out of storage on the SD card (4GB would take a long long time to fill!)