Wheelbase will depend on your height. Plan explains how to set the overall length (and therefore wheelbase) based on your leg length.
Yes, 20 inch wheels. For a non-leaning bike (i.e. 3 or more wheels) these are stronger than bigger wheels.
Track: I can't recall without looking up and hesitate to publish plan details publicly. A quad can be narrower than a trike without sacrificing stability and the Streetfighter is narrower than the AZ trikes.
If you're building it then you can adjust whatever you like, combine bits of several plans, etc. Recommend that you don't adjust the shape of the wheels - round seems to work best
On the old forum, Fantom's nephew is a great velomobile based on the Streetfighter:
Hi all, Sweden calling! To gently increase pressure in the building process I'm hereby establish a thread on my velomobile project. Don't need to argue about the "fun" part in this community, I guess! When it comes to "practical" my starting point looks as follows: I commute 8 km to...
There are also several YouTube videos for Fantom's Nephew.
looking everything over, needing to order some more plans, but I am leaning towards a delta trike configuration with a simple carbon fiber body over it. Either the Aurora or the Delta Wolf probably the Aurora. The Streetfighter seems to add needless complication to a Velo that a trike doesn't have.
My plan is to build the trike than carve the body out of foam blocks over it, cover the foam with carbon fiber epoxy mix, then remove the foam. The drawings are more of a guide than anything.