I got tired of falling over when climbing off my old bike fully loaded with cargo. I looked for one that had all the things that I needed, like easy dismount , full MTB gears (for climbing hills with cargo), and a front rack that would not affect the steering when fully loaded with 50lbs or more. I could not find one at any price.
At the same time someone told me that the bike was too short for me. So I moved the seat back by making a special seat post, and I discovered that I could sit up straight (to relieve my carpel tunnel syndrome) and still apply my weight to the pedals, because the crank was slightly forward. 4.75 inches is 60 degrees from the seat; just like the old bikes made for sitting up straight. I still need to lean forward a little to climb hills. This crank forward also encourages me to pedal in circular motion.
So I worked a long time measuring and drawing to invent this one. And it took me a couple years to eventually build it.
When I was designing recumbent bicycles I discovered the ergonomic power angle. When you want to design a bike just for you, measure for a 40 degree or more when climbing hills and a 30 degrees to the back of the seat for cruising. You can move around on the seat to get the right position.
Recumbent bicycle seats are usually set at 35 degrees with 5 degrees of adjustability.
If you use a 24 inch wheel it will be like a lower gear that is good for hill climbing with cargo.It is better to have a lower gear than you need so that you can spin the crank fast, like bike racers do when climbing.
If you use disc brake in the rear and have a steep steer tube angle, you could switch wheels later and still have a reasonable steer angle.
You can build one of these fro around $400. Do the math your self, this editor keeps loosing my list of parts.
To calculate seer angle try this one http://www.kreuzotter.de/english/elenk.htm
Find more info on bicycle design on my non-profit blog abut Utility Cycling Technology: