From Huddersfield Canal Society: http://www.huddersfieldcanal.com/canal/boating.htm
The official maximum dimensions for the Huddersfield Narrow Canal are:
length 70' 0" (21.3m), beam 6' 10" (2m), headroom 6' 2" (1.8m), draught 3' 6" (1m).
Craft with a beam over 6' 10", such as some historic boats, have experienced difficulty getting through locks in Uppermill and in the Marsden flight.'
Frankly, knowing how frequently water levels are not maintained, along with the various items which find their way into canals (traditionally bicycles and shopping trolleys) I would think that the smaller the draft you can get away with, the better.