It all depends on your entering tap water temp, if its cold enough then 6m should be ok. My chiller was made with 9m of 3/8' as thats what I had left over, the cold water entry/wort exit is on the bottom and the wort entry and waste water exit is at the top.
The outer shell of the chiller is at about tap water temp up to the third coil from the bottom (see the arrow) which roughly equates to 6m of useful cooling in the top 6 coils, the lower three are not really doing all that much. I plan on making a prechiller coil for the entering water to sit in a bucket of ice so I can get my leaving wort temps down a bit more in summer.
My CFC is 6m long and does the job just fine. In winter it gets the wort well down below ferment temps. In summer I have to slow the flow a littl and thus it takes a little longer but still works fine.
You will use up 35L of water pretty quick through a counterflow.
Better to crush the 2l blocks of ice up in a bucket with a little water so you have a nice slurry, sit your immersion coil in it and run your incoming water through it. Have a search through the thread called Perth brewday to pics of Goats setup. He runs an immersion coil in the kettle with a large plastic crate filled with 10L frozen plastic water bottles in it. It takes a long time to get the ice in the bottles melted.