Its been a while since I was involved in a abnormal load transport in France (as i mainly specialise in the UK & Ireland). The last time I went the catorgorys were as follows:
CAT 1 Permit - >20.00m long x >3.00m wide x >48,000kg. A pilot car isn't necesarry (normally). A Cat 1 permit is required but these come in two types, either a one off permit or a roaming permit. A company like CPL or Interboat from Reading will proberly already have a roaming CAT 1 permit.
CAT 2 Permit - 20.00m> long x 3.00m> wide x 48,000kg>. Normally a pilot car is needed for this catorgory.
CAT 3 Permit - 25.00m> long x 4.00m> wide x 72,000kg>. Normally a pilot car (or two) and the police.
As I said, these rules may have changed, I know the french system has been made some changes recently. However the transport company will be able to tell you the up to date rules and also sort it all out for you.
Of course, you may be able to put the boat in the canal at Calais without needing to drive out of the dock(not sure if this is still possible), thus, not needing any of the above.
Rob.