10 metres for each rein should be long enough, i got 15m for mine but i cut some off for lead ropes. Also, get different colours or in this case white with different colour specks, it makes it much, much easier to see which side is which and saves time if you do end up tangled.
I got some IV horse ones at christmas that have rope at one end so they slide through the rings on a roller better, but then are like normal webbing reins at the other ends, love them. Not sure what length they are though.