Size is not the issue (as such), it's all down to the buoyancy of the top bait compared to the density of the bottom bait. As they can vary there is not a set formula based on size alone.
Just experiment and try them out in the margins to get some idea of how they might sit when cast out.
Not sure about the colour mate
Bedded on well , was hitting the clip at 140 with it yonny, in cross winds so can't moan at that . I can tell it's got the abrasion properties you mentioned too, did a little test & it can take some real abuse before it gives up,
Thanks for the heads up
That colour though
There is no standard but the most common are a size 12.
Size 11 seem to be popular too nowadays (both Korda and Gardner have moved from 12 to 11 I think), along with the smaller sizes 8s and 9s.
The size 12 is what I consider to be a standard ring swivel and will fit most heli set ups.