hi,
I have a dash 10 or a dash 12 for the rb26. As for size it would depend on the catch can fittings or your personal preference i think. The dash 12 is bigger by about 2mm in diameter and will flow a little more.
I think the dash 10 is enough and have not had any complaints with this size. I have had a request for a dash 8 for a rb26.
So short answer i would say its personal preference and also what you are connecting to.
Thanks