|I had to say it...|
Honestly, it depends where you are racing. If you were racing in south Florida, I would say don't even bother screwing in the spikes in the first place. Even so, if you are in the Pacific Northwest, in like Washington or Oregon, I would say 1/4 inch is fine. You may want to buy an extra package of the little metal piecessets of christmas tree spikes. The pieces of metal usually only last me 6-8 races before they are missing/warped.