depends on how much gear, how much freeboard you're comfortable with, are you paddling in flat water or do you anticipate waves/wakes from other boats, etc.
a good starting point is to consider a pirogue's displacement as two triangles whose base is at the midpoint and whose height is half the length. The area of a triangle is 1/2 X base X height. So if you put the two together, the footprint area for a pirogue is the midpoint times 1/2 the length. A 14' long pirogue (bottom length) with a 2 foot wide bottom would have a footprint of approximately 14 square feet. If the sides were vertical and you went down one inch, then you would displace 1.17 cubic feet ( 1/12 ft X 14 sq ft = 1.17 cu ft). Water weighs 62.4 pounds per cu ft so one inch would displace 73 pounds.
Most pirogues do not have vertical sides so displacement increases as the pirogue is pushed further into the water. If you can cut to a line, two sheets of plywood will easily build you a 14 or maybe a 15 footer
