I used 50 ft of copper tubing wrapped around a corny keg to make my IC.
I typically brew 10 gallon batches in a 20 gallon SS kettle.
The top of my coil tends to stick above the wort (no big deal) by an inch or so.
Regardless, it still chills 10+ gallons in under 15 minutes to ambient temps.
If I had wanted more of my copper tubing immersed in the wort, I could have used a cylinder with a larger diameter (e.g., a 5 gallon Homer bucket). However, when I was contemplating my design options, I was thinking that, long-term, I might like to use this IC as my "pre-chiller" and be able to place it inside a bucket or cooler of ice water--hence the tighter coil.
I made a handle out of PVC pipe, wooden dowel fillers, and black electrical tape to avoid burns. It makes manipulating the IC (i.e., whirlpooling) and lifting it out effortless and safe.