Generally, stated mature widths are at the widest point, which is commonly the top or canopy of tall grasses. Obviousy, everything depends on the form of the plant, and there are many different forms and habits in ornamental grasses.

Spend some time looking at the pictures of different grasses on Bluestem's website, linked below. They provide a great deal of information, as well as a comparative chart on many grasses.

Here is a link that might be useful: Bluestem

Thanks. They have some great looking grasses!