Does it have to be standing?

Either way, dunno if these are owt like the tallest/longest, but they're the biggest I've seen:

Standing - Dol menhir (9 metres)
http://www.janetomlinson.com/images/photographs/france2005/1dolmenhir.jpg (also Kerloas http://www.mega-lithes.com/sites2.htm which is a fraction taller & I didn't know about when we were there.....)

Fallen - Le Grand Menhir Brise (difficult to find a pic that gives any sense of scale, but it's 20 metres long)
http://www.megalithic.co.uk/modules.php?op=modload&name=My_eGallery&file=index&do=showpic&pid=3475&orderby=
and again
http://www.educnet.education.fr/obter/appliped/morbihan/gavrinis/gavrinis.htm

love

Moth