C-C means 'centre-to-centre'. You measure the top-tube starting at the halfway point (laterally/horizontally) of the seat-tube ending at the centre point of the head-tube (see diagram below - it is measurement B).
http://www.ribblecycles.co.uk/assets/im ... rAudax.gif
http://www.ribblecycles.co.uk/assets/im ... rAudax.gif