D03 Valve
D03 valves can typically handle flow rates up to 13 GPM. There are some models that can fall upon either side of this rating. This can be attributed to the manufacture or spool choice. Measuring the port will yield a diameter of around 1/4".
D05 Valve
D05 valves can typically handle flow rates up to 40 GPM. There are some models that can fall upon either side of this rating. This can be attributed to the manufacture or spool choice. Measuring the port will yield a diameter of around 3/8".