In reply to montyjohn:
> Indoors 30m is plenty, but outdoor top ropes could be anything.
Well...that very much depends on the indoor wall. My local wall is over 20m high so even a 40m would see you failing to get to the floor or, even worse, with rope not knotted to a rope bag, you being dropped. So, it's not good advice to a novice to just say 30m is fine.
To the OP I'd personally go for 60m then, as you later cut worn sections off either end, you'll still have 50m+ left. Anything 9mm to 10mm. I wouldn't bother with dry treatment - not worth the extra expense.