Here is what the NEC handbook says about static in spray booths:
"To prevent sparks from the accumulation of static electricity, all electrically conductive objects, including metal parts of spray booths, exhaust ducts, piping systems conveying flammable or combustible liquids or paint, solvent tanks, and canisters, should be properly grounded. See Section 4-5 of NFPA 33 and Section 9.3 of NFPA 77, Recommended Practice on Static Electricity. "
Section 4-5 of NFPA 33 requires bonding and grounding, and Section 9.3 of NFPA 77 warns of the danger of static discharge. But the Appendix A 4-5 references NFPA 77 in its entirety.
126.96.36.199 of NFPA 77:
"Where wire conductors are used, the minimum size of the bonding or grounding wire is dictated by mechanical strength, not by its current-carrying capacity. Stranded or braided wires should be used for bonding wires that will be connected and disconnected frequently."
The NEC, (NFPA 70), Section 516.16 requires the bonding jumper to be sized according to 250.122 and no smaller than #14 AWG.
But, you already know all this.
I would bond the metal parts of the electrical installation using table 250.122, and let the user worry about bonding the portable stuff.
If the customer has hired you to do the jumpers then: Appendix C of NFPA 77 has several pictures on how this is done in spray booth applications. They recommend 1/4-3/8 inch flexible bronze or 1/8 inch stainless jumpers and clamps, among other sizes and methods. Pick and choose a method that works for your situation.