For wire size it depends on your over all power draw. I would use either 10, 8, 6, 4 depending on what you are using for your feeder and it's amperage, it's normal for the ground to be smaller than the conductors.
The Wegner Versastage will bond to each other already as they interlock.
Yes, lighting trusses should also be grounded as well.
Yes to bonding the stage to the incoming ground (EGC - Equipment Grounding Conductor)
Yes to bonding the lighting truss to the EGC as well.
The size of the ground/bond wire required is a function of both max circuit breaker capacity and lightning protection. So you should match whatever size ground wire is coming in from the POCO if possible, but probably not smaller than 8 gauge for lightning protection. Also, avoid sharp turns with this bonding wire if possible (no right angle bends) since that will promote lightning side flashes. That's in the amateur radio manual, and those guys worry about lightning a lot.
Yeah, we don't like to think about lightning, but by definition these shows are outside in the summer and typically the highest structure in a flat field. Even if you evacuate the band and stage crew when lightning rolls in (you do, don't you), your equipment will stand a much better chance of surviving a nearby lightning strike if the stage and lighting trusses are properly bonded to earth.