tominboston, I just went through this myself - I'm slowly putting a keezer together. It appears that some people use 1/4" tubing and some use 5/16" for the CO2. For a small homebrew setup either should work fine. I think you want the barb size to match your hose size (ID) either way. Some places refer to the 5/16" as "gas hose". You can get it clear, red or blue. I went 5/16" in red just so its easier to tell the gas lines from the liquid lines when its all stuffed into the keezer.
There are several types of tubing, so I discovered while searching. I found this page helpful.
I think sneezles61 is talking about Oetiker clamps. They come in various sizes. Note that you do need a special clamping tool to install them and they are intended to be permanent (meaning you destroy them to remove them).