In reply to Alex Pryor:
I've got 6mm I think, I'd be happy using 5mm, 4mm might begin to scare me a bit.
According to Needlesports website, 4mm cord has a 4kn strain, take about half that off for a knot (down to about 2.5kN say), and then I've read somewhere that a jerky abseil by the average male abseiler can cause up to 1.5-2kN of force, so the safety margin is under 2, which doesn't leave a lot of leeway if things go pear shaped. 5mm cord breaks at 5.5, so youve got a bigger margin. Although if its an emergency, 3mm cord takes 1.8kN, and so you could theoretically go off that and abseil as smooth as possible (although I wouldn't be keen myself).
The other thing, is is there a radius at which it starts to damage the rope - loaded rope over a wire will cut through, but what diameter cord will have the same effect - I'd guess that would start to kick in at about 3mm cord?
AJM