Pick the trailer, type the size your customer wants and what it has to haul. You get the 3D drawing, the cut list, the steel to buy, the axle, tires and coupler it needs, and the price to charge.
The trailer is only as strong as its weakest part: axle, tires, coupler, hitch and tow vehicle. Use the rating stamped on each part, and check your state's rules for brakes, lights and titling a homemade trailer before you sell it. Tire loads: published ST tire tables (Dexter/Goodride, Carlisle) at max cold pressure. Tongue weight 10–15%: common guideline for bumper-pull trailers (vehicle owner's manuals, GMC).