Vetruck had posted the answer to this question in his handling thread a while back, but ofc that thread isn't worth its weight in data now because for some inexplicable reason all his posts are gone.
Anyways, does anyone recall what size bars he recommended, or know enough to suggest the best size to go with for a v6 auto-x car?

He always said the sway bar should be used to fine tune once the rest of the suspension was set up. Plus your own preference as to over/under steer and roll. I like flat and a tendency to over steer. I've got 36/23mm and like it. I've heard others that prefer a lot less and some even with no rear bar at all. If you can get a selection and try combinations to find what works for you.

With stock springs? If so as big as you can get your hands on. With the poor camber gain up front and negligible camber-loss in the back the car will understeer less with a street-friendly alignment and a bigger bar. A big front bar should help put the power down in the back as well. I'd go with the 36mm up front, get at least 1.5° of negative camber, max caster, and play with rear sway bars as needed to balance the car. If you need one of the larger rear sway bars you'll probably start running into wheel spin issues even with a v6.

If you can get enough negative camber up front then ignore this and use the sway bars to tune accordingly.