On the rods I use most frequently, I normally have a semi-permanent leader butt section nail-knotted to the line. I use perfection loop knots to connect the rest of the leader.
Depending on the line weight the diameter ranges between 0.018" - 0.025" (about 0.002" larger than the next section of the most frequently used leader).
Length varies with the rod action. With a new rod, I start with about 3', try it, and then cut it back until I get a good "turn-over".
For the rest of the leader, I generally use the formulas published in McClain's Encyclopedia, but if you search this forum for leader discussions, you'll find lots of different (and valid) opinions on how to make a good leader.