1/4 inch seems to be the preferred size. Not too large but it doesn't cut into your hands.

There is an Aussie club site that addresses safety requirements for the boats in their organization. They require the perimeter line to be secured every 75 cm (about 30 inches). That seems a bit close and I can't figure out what a close spacing accomplishes. I put perimeter lines on my boat and secured it as needed. I think my greatest. distance was about 38 inches. I don't make the line taut -- I want to get my fingers under it easily.

There seems to be some difference of opinion on running the perimeter lines at the cockpit. The cons talk about entangling yourself when you are getting back into the cockpit. The pros say they have never had a problem and it is necessary when helping a swimmer. I am with the pros. I have done a lot of rescue practice and never had a problem.