T-Tedd, there are many folks here with experience designing/building tadpole recumbents. From what you said above, I'd guess that you are more interested in a touring style ride than a racer, and since you are planning to motorize it that weight isn't a huge issue. Twenty inch wheels are generally stronger than 26 inchers, given the same spoke gauge. Answering the question of which is stronger requires that you address a lot of variables. 26 inch wheels with large hubs to accomodate a larger diameter half axle and 12 gauge spokes would be more than strong enough unless you are very heavy. From reading your various posts around the forum I don't think that is true.
Seat height and BB/crank height are matters of frame design, and can vary greatly. You mentioned possibly making the rear drive wheel suspended - there are several approaches to achieve that. Since you mention 1 inch square tubing for your frame, I'd suggest you do the dynamic loading analysis, as that is fairly small. Heavy wall one inch might work well, but 1.25 to 1.5 or even greater tubing size is more commonly used.
Seat design impacts the other design elements heavily - most tadpoles contemplate a fixed seat position with a variable angle, with an adjustable length crank boom out front. It is quite possible to go to a fixed boom with adjustable seat position to change the X seam dimension, or even to combine variability of both seat position and boom length.
To my mind, the critical questions are: 1) intended use (short rides, day trips, extended touring); 2) desired load capacity, which of course includes yourself as the primary load; 3) desired control input interfaces. Given my own physical limitation, side steering isn't terribly practical for me, whereas for most tadpoles it is the commonest type.
Since you say you plan to motorize the bike with a Honda engine behind the seat, the design requires there be sufficient room for that engine and still allow you some back rest angle variability.
The links Klox provided are excellent references.
Zero point angle is a function of several factors - width of the front hubs plus disc brakes, if you choose to go that route, and wheel diameter. What you are shooting for is, if the axial line of the kingpin is extended to the ground, it hit the ground in line with centerline of the wheel once mounted, parallel to the bikes length.
So, from an analytical point of view, you must first establish the width of the wheelbase and its length. Once you have those numbers, you need the front hub width and the intended wheel diameter with tire mounted. Figure the zero point angle, use that number to calculate your suspension member sizing, and the required angle of rise or drop to your main frame member height above ground.
Like I said, lots of variables.
