Since you may be asking, which spindle is right for me? Here's a brief description of the five sizes and how they may be typically used.
Sirocco - smallest at 1.125" inches, very fast, but short spin time (typically 30 seconds) - great for thin cotton singles, silk (I've been spinning mawatas on mine and love it for quickly putting twist into predrafted fiber), cashmere, yak, camel down - anything you want superfine.
Mistral - 1.25", more of an all-purpose spindle for fine singles, great for cotton, and other short staple fibers such as merino, cashmere, yak, camel and fibers that need a lot of twist such as silk. Typical spin time of 45 seconds.
Squall - 1.5", middle of the road spindle - can still handle short staples with ease (I spun some cashmere from the cloud with one) but can start adventuring with more typical wools like merino, alpaca, cormo. Typical spin time is around a minute.
Chinook - 1.75", starting to get into the Tibetan-like range, I've only done some test spins with them, but have had no problem with wools making reasonable singles. Typical spin time is 90 seconds.
Zephyr - 2.25", longest and slowest spin time (typically 2 minutes) definitely in the realm of Tibetan spindle in behaviour, probably happiest with wools (even longer staples such as BFL), but can spin thicker cotton singles too. Have even heard reports of people using it suspended as a bottom whorl.
In general, spindle speed is inversely related to spin time. I test spin each spindle multiple times throughout the making process to check for any signs of wobble and lack of a sustained spin. Because of the carbon fiber shaft (which is very sharp!), it is probably best to use a glass or ceramic bowl with these support spindles.