If this is your first festival outfit: start with a bodysuit. One piece, one decision, nothing to adjust while you are dancing. Everything else is styling on top of it.
If you are on your feet all day: check what the straps are doing. Cage strapping looks incredible and digs in after six hours. Wider bands and lined edges are the difference between wearing it and carrying it.
If it needs to survive more than one weekend: buy for the hardware, not the print. Rhinestones and holographic film are what fail first, and sewn-on beats glued-on every time.
On sizing: read the measurements, not the letter. Much of this is one size or a close-cut S, M and L, and the stretch is doing most of the work. Plus size rave and festival wear is cut for extended sizes rather than scaled up badly.
If you want it to work after the festival: the mesh and strappy sets are the ones that do. That is the point of shopping rave and lingerie in the same place. A cage bodysuit does not know what month it is.