In reply to BB95:
Could be wrong but I think 8mm is usual off the shelf size, but you can get 8.5 and even 9 mm. Nothing stopping you taking any half climbing rope or even full though if you wanted to. It all depends on the circumstances and the likely use. I have often used a 8.6 mm 50m half rope as a confidence rope just cause it suited, but also have used 30m of 7mm accessory cord for basic confidence rope situations as it's easier to carry and pack.
Post edited at 17:05