There are an endless variety of shapes to use. Traditional touring paddle blades typically fall within a 6" wide by 27" LOA (tip to throat) range. Stay within those parameters and you'll have a nice blade. Make some more paddles before you start getting more specialized as function will dictate form.
Draw out the shape that is in your mind's eye, refine it till its sweet, then fold it in half lengthwise and and use one half as your pattern.