Anthony, thanks for your detailed reply. All your assumptions are correct except for the load which, if I understand the truss specs correctly, is only 43.3 psf.
Anyhow, I went to the city and they said any lumberyard could do the calc as well as supply engineered drawings (which appear to be all computer generated these days anyhow). The local yard I went to had them within minutes and recommended an engineered beam of 3" x 10". However, since I want to temporarily enclose this each winter I'm thinking I just may add some additional posts so there's something to attach the wall material to. However this is a lot more work (which I was trying to avoid with only 2 posts) since it involves cutting into an existing poured slab in order to install the sonotubes and poured piers. Nothin's ever easy !!!!!!!!!