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20% isn't really enough for anything but quite flat moulds. I would go atleast 50% to 100% larger than the size of your moulds footprint (including the flanges). 100% bigger will be big, so I generally only do that for very deep moulds or quite complex ones. A canoe is a fairly simple shape, but has quite a deep draft - I would recommend somewhere in the vicinity of 50%-65% bigger. Remember that there is no harm in going larger other than wasting material, but going smaller is a big problem, so 'err' on the larger side.
As for pleats, this is always difficult to describe in words, but my best advice is to start at a known location. If your flanges are rectangular and have corners, start at a corner. If it doesn't have corners and is more like the 'oval' shape of your canoe, this will be a bit harder because you will really have to cut your bag to suit the shape otherwise the overhang may confuse you and you'll end up with messy pleats. But in this case, start in the middle and work your way towards on end of the mould. It is important to start in a known location, because if you just start anywhere then you can lose track of how much bag you have left and can make the bag too small in one area.
For pleat sizes, again there is no harm in going bigger other than wasting bagging tape. If you aren't confident of how big they should be, just make them quite long, like 4-5" each. You can always fold them over onto themselves more if you find you only need short pleats, but you need your pleats to be long enough to take up all of the extra bagging film you have. Placement wise, I tend to place them every 4-5" as well as at every change in geometry of the mould, ie. long straight sections can get away with less pleats, lots of geometry requires more pleats. That isn't too critical, as you work your way along the flange you can decide whether you need more pleats and just add some more.
Realistically, the size and placement of your pleats is mainly determined by the size of your bag. So start with making a larger bag, and the pleats shouldn't be too much of a worry.
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