Firstly
It all depends on what you want to target lets say you want to target edible fish then a leader of between .80 and .90 will be fine if you want to go for sharks then a leader of 1mm up to 1.8mm will be good.
You cant use too light a leader as your mainline is normally between 13-20kg breaking strain, you need a thicker leader for that extra protection incase something decent picks you up and you fishing in a rough area, length wise if you fish for a club it can be no longer than 8-9m long if i'm not mistaken.
I normally double the length of my rod with a couple of turns on the reel.
Hope this helps