I use a 15m round bio thane in (I think 6 mm). Wish I could have got a thinner version but they were out of stock. 15 metres is better than my previous 10 metres, it just gives you more to work with when going through thick early stage coppice, which I have a lot of on my ground. but 15 metres of 6 mm is a lump in my roe sack.
Use it with a niggeloh tracking harness which, is heavier that the Artus harness i used previously, but I found that the Artus harness attached the leash to far forward resulting in tangles if the dog spun on a spot. If I could have my time again I'd get the niggeloh with the 2 buckles which would make the slipping of the dog easier.