Hi Earthstepper,
I don't think the Peterborough Stone qualifies as the smallest standing stone in Britain...
http://www.themodernantiquarian.com/image.php?image_id=19238
Here are my contenders for the coveted title...
Castell Coch
http://www.themodernantiquarian.com/browse.php?site_id=4081
Garreg y Big's little brother
http://www.themodernantiquarian.com/image.php?image_id=9328
I reckon there has to be something even smaller out there.
Kammer x