There are all sorts of shapes and sizes, as well as many different ways of building canoes. there are no wrong or right way of building one. It depends on what you want to do with it, the waters you'll be using it on , etc
. In the end you can just follow your heart!
Also, depends how much time and money you are prepared to spend on it. I have built flat bottomed ply ones in less than a week, skin on frames and strip planking, taking many weeks of work. They are all valid in their own ways. 3 metres is a good size, although it can depend if you are 5' tall and weigh 8 stones or 6'5" and weigh 20 stones
. Perhaps you need to narrow down what your requirements are before you decide what you are going to build.
This one is a skin on frame. Pictures not very good
The frame were steamed oak. Very light canoe, nice to paddle, but a bit too narrow for me. I am 6'3" and too heavy!!
That one a strip plank and veneer skin.
I built them without plans, using some line drawing in The Bark Canoes and Skin Boat of North America.