Civil 3D 2010 changed the way structure descriptions are populated when structures are created. The description used is the part family. In the past the description included size information.
The easiest way to get the old type description (after the initial setup) would be to create a custom pipe rule that would set the desired description. I’ve done some posts in the past that have covered how to create custom pipe rules, an AU paper in 2007 and you can also find some information in help.
As indicated in my last post this idea won’t work for Null Structures.