Even though the t-shirt yarn class I am taking (which ends next Monday) comes with four custom (and customizable) patterns that Stefanie designed specifically for the class, I wanted to swatch some of my yarn just to get an idea of how it looks. I knitted up one of the balls on size 10.5 needles:
The lightest weight yarn you can really make without breaking the material is about a heavy worsted. I would say what I have knitted up here is a bulky. The most WPI I have been able to get is 6.5, and this yarn is about 5.5 WPI (wraps per inch - wrap the yarn around a ruler and it helps you get an idea of yarn weight - fewer wraps = heavier yarn).