Step 2: Assess Your Needs
Take stock of your physical dimensions in order to build a suitable frame. If you are over 180 lbs, you will likely need to incorporate beefier tubing than the 1" tubing I used, both round and square mild steel. Or you could add gussets, truss pieces, or use other ways to stiffen the longitudinal aspects of this rather long-wheelbase trike. Some flex feels good over bumps, that's a bonus of a long wheelbase. But undue deflection could lead to eventual failure for heavier riders. Think "custom" frame, and adjust to suit your build. As for length of the cockpit, it's good to have some room for adjustment. At a minimum, your feet need to clear the front cargo deck. If you are of moderate leg length leave extra room for that taller friend who will want to try your machine. You can have some flexibility by making the crank mast and the seat points moveable on their mounts in one way or another. Accomodating a rider with longer or shorter legs than yourself will entail changing the chain length with my current design. Incorporating additional chain tensioners can free you from that, but adds to cost of parts and adds a little bit of friction, noise, and weight to the drivetrain.
A basic starting point is to measure your sitting leg length: Sit with back against a wall with legs straight out, and measure the distance from the wall to your heels. This will define the general seat back-to-pedal parameter. Other things like height of canopy, width of steering bar, etc, you can work into the design to suit your build as you go. With this triangular frame design that narrows from front to back, be sure your heels will clear the side tubes as you pedal.