Chambers are easy to size. Runners take a little trial and error. If you want to run 12's and under, 200-220 cc runners will probably be ideal. Give Air Flow Research a call, they make some of the best heads and will point you in the right direction. For the chambers, smaller is better, but so are flat top pistons. You want to run the smallest combustion chamber you can along with a flat top piston while still meeting your target compression ratio and having enough room around the valves to get good airflow.