Originally Posted by ACarver
...20' sections of 3/8" copper pipe....
Pipe? Make sure they aren't (rigid) pipe. You cannot coil that stuff, it will fold up.
Or are they "coil" remnants? 20' is not a common length for (hand bendable) coil stock.
Also there are 3 main wall thicknesses in copper, K being the thickest and M the thinnest. L is in between the two and the most commonly available. There maybe some M coil out there too, given the current price of copper, but I'm not sure how that holds up to bending relatively tight radii without special bending tools. Any should work for a chiller coil, but the thinner the wall, the better (faster) cooling you'll get. So there is the trade-off between workability and performance.
I agree that 1/2" will give a much faster chill. So 25' of 1/2 inch may give similar efficiency as 50' of 3/8".
And soldering copper is not that difficult. Look on YouTube for some demos if you feel the need.