I got a tremendous deal on some 5/8" a while back and made a dual pass IC, the water goes into the bottom of both coils, and the water comes out the top of both coils; basically a "rib cage" design, and it cools 5 gallons fast. It is 42 feet long and works perfectly in a 30 quart kettle, as it is tall and narrow, but it is useless in a 40 qt or larger pot, as too much is above the level of the wort. 5/8" is a real b!tch to bend w/o kinking. I would suggest using the largest form size you can, so if you buy a larger pot it is still effective. It dawned on me why most of the pre-made IC's average 10" in diameter; they will fit into almost any size kettle. I have a 50' roll of 3/8" waiting to become an IC, I just haven't decided on a design yet.