Mark, there are no hard and fast rules when it comes to twisted leaders. The length range between a one metre twisted leader to some that are four metres in length. Some poeple use it at the end of a very elaborate rig involving spliced assist line where the twisties is quite short but locally, you'll find most people here fish it with the twisted leader being just on the spool when you're casting. That would make it some three metres of so in length.
There is a school of thought out there that the twisted leader is a primary cause of wind knots if it is too long. I think there is some fact in this and these people are using a shorter twistie which is about two metres long so it is in the guides, not on the spool.
In terms of diameter, you could use 200lb mono. It just wouldn't cast very well! I use 100lb Penn 10X as my standard material but I will go as high as 170lb.
