Regarding sizes of washers, #6 is just small enough to not have typical spoke heads pass thru them, but not small enough to actually use--under tension the heads will deform the washers and pull thru anyway. They *might* work for some spoke nipples though.
#8 would probably be too big for the nipples, and is definitely too big for the heads.
#4 might be small enough for the heads but I think they'd still be too big.
#2 is probably a perfect fit, but I haven't found either #2 or #4 locally to try them. I dunno if the odd# sizes exist but if they do maybe they would work even better.
(I tried the #6 on a Crystalyte HS3540 hubmotor, using the Grin Tech Sapim spokes, because the flange holes on the HS are HUGE and can pass the whole spoke head right thru them.
With the washers it's a little better, but I can only lace the wheel with the heads out on the flange, not heads-in like I wanted to for a "wider stance" of the spokes for what should be better side-loading ability.)
Oh, also, stainless washers bend pretty easily, so I'd recommend heavy duty ones, like the Grade 8 or something.