It's easiest to make one that goes out to the mounting holes... Usually 2" by 3".
However... as long as it's touching the entire core surface, then it's fine. I ounce saw one that was just a square roughly the size of the core that they had thermal epoxied on to the chip. Worked well for the time. I know it was on an Athlon. Basically it was a 90 degree bend.
The one I'm making will be 2" by 3" but very slimline. Copper will be here Wed, but may take a few weeks to get aroung to making it.
Edit: Not sure what you mean about water dissapearing. That shouldn't happen in a closed system. Some people use eveaporative type cooling (bongs) and they do loose water. In a closed system if you are losing water something is wrong.