I hate chalk. HATE chalk.
That being said, I've determined what my favorite kind of chalk is: white, about an inch and a half long.
There are reasons for this. First, the "blackboards" here are actually green. The color options I am given for my chalk are white, blue, pink, yellow, and occasionally teal. Blue, yellow, and teal tend to blend into the background, making reading difficult for kids in the back, or at angles with a glare. Pink just makes me feel really girly and biased, not to mention with the green chalkboard it reminds me of childhood shows with a giant dinosaur (okay, that was purple, but it was a pink-ish purple).
Secondly, chalk dust goes EVERYWHERE. I often wear black skirts to work, and while the rainbow effect is pretty, I stand out enough as it is. White chalk dust is a little less attention catching.
Now for the size. Why an inch and a half? Anything shorter is difficult to use, or will wear down too quickly into a size difficult to use. Anything longer I will break. Belive me, I've tried. I've tried using a whole piece of chalk. I've tried using a two inch piece of chalk. Perhaps the chalk is inherently weak (*cough* made in China *cough*), perhaps I just don't know my own strength. Whatever the case, it breaks consistently enough for me to have given up on it.
So there it is, folks, the part of the teaching experience I'm sure you have been the most curious about! Consider your unasked questions answered. You're welcome.