8mm is perfectly fine Mark, but it's harder on the hands when hauling it up. That's why I went for 10 myself.
As for the length, I think I bought 25 metres, as my winches are just behind the traveller on the coach roof. The extra gives you the ability to end for end it later on if there are any wear points. That being said, mine has been there now for four years, and apart from some fading of the red to a lighter shade, there is no sign of wear.