There are 2 considerations in the boat design: making sure it floats and making it as fast as reasonably possible.
To float, the boat has to have enough volume to support your mass. Suppose the boat is for 2 people with a combined weight of 250 lbs. or mass of 120 kg (it's easier to deal in metric). Since the density of water is 1 g/cm^3, the volume of the boat has to be at least 120,000 cm^3. A resonable length and width might be 200cm x 75cm. So the depth has to be at least 8cm, although this would just barely float. I'd triple that and make it about 24cm.
So now you have a box that's 200cm x 75cm x 24cm (or about 6.5 ft long, 2.5 ft wide, and 10" deep). That's not a terribly efficient shape. There are a few things you can do:
1. Make it longer and narrower, but keep the volume the same or larger. So if you make it 10% narrower, you have to make it at least 10% longer, too.
2. Make it narrower at the bottom than the top, giving the hull more of a V shape. To keep the volume the same, keep the average width the same. So the bottom of the boat could be 2ft wide if the top is 3ft.
3. Make it narrower at the front and back than in the middle. Keep the average width the same or increase the depth to keep the volume the same.
When you paddle it, I'd suggest kneeling in the boat. There probably won't be much space for both of you to sit comfortably and paddle effectively. With 2 people, put the lighter person in the front. Good luck!