I have both 7 mm Polilite & the RS sourced 7 mm Swiftchord.
I found I lost grip on the Pollilite at crucial moments, it's outer is quite smooth.
I've not experienced the same with the Swiftchord to the same degree.
For length I suggest you need to be able to get the boom back from 90 deg whilst sat well back and out; 10 m is what I started with, and I haven't shortened it yet.