Gather up a bunch of scrap cardboard and lay it on the ground under the trike. It should extend past all the corners of the trike. Tape or otherwise secure the trike to the cardboard, so it doesn't move while you are taking measurements.
Using a carpenter's square and a level, marked out the following dimensions of your trike. I simply drew the "outline" on the cardboard, then measured them later with a tape measure.
- The very rear edge of the back wheel
- The front edge of the rear wheel
- The center of the rear wheel
- The furthest forward that the pedals will extend
- The center of the BB/pedals
- The very outside edges of the front wheels
- The front and rear of the front wheels
- The maximum and minimum outside extension of the front wheels when fully turned
- The width of the seat
- The height of the seat
- The lowest point on the trike (frame, chain, chain hanger, etc)
- The maximum possible height of your knee when seated, with feet on pedals
- The height of your shoulders when seated
- The height of your eyes when seated
- The top of your head when seated
- The width of your shoulders
Based on these measurements, you can determine how wide the velomobile needs to be, where the cutouts for the wheels should go, how tall it needs to be, and how long. If you don't want to design from scratch, you can easily resize, stretch or move various parts of the model I made to suit your trike. For instance, if you are shorter than I am, you could lower the hood and pull in the nose a bit. You may end up with a few extra folds here and there, however.