My shop has outgrown it's spray booth (protective coatings) and I need to build a new one.

The booth will be approx 12 high, 23 deep and 30 side to side (i.e. opening) and needs to be a clear span, devoid of supports as well as no supports within the structure..

In an area approx 10x3 at the front near the opening will be a exhaust system weight in at approx 500> lbs (dead load), with the majority of the weight in the center about a 3 foot diameter circle. Beneath the 10x3 are will need to be openings to suck air up into the aforementioned plenum (10'width, 3'depth, 2'height). Other than the installed fluorescent light fixtures, there will be no other items or weight stored on the roof nor will there be any live load of any type.

I really have two question. First is what will I need to support the span and in addition what would be the stud size recommended for the walls?

There will be at least 1 standard size door installed on a side wall with possible another on the other side. I generally overbuild anyway, so have at it.