62 feet, but it's easier and dryer if you're shorter!
At full length for Wigan Junction to Leeds you need to make sure your boat can cope with huge amounts of water coming through lock gates.
A good cratch cover, or very big scuppers at the bow. A trad stern with watertight doors (best!) or a cruiser/semitrad that doesn't let any water through deck boards, or at a push a few rolls of gaffer tape to seal the deckboard joins ...
57' or less and you'll be fine if you pay attention and don't mind getting splashed!