I'm having a similar issue but I'm farther along. I have a 54" inside diameter eccentric cone sanitary sewer manhole with an associated style. However, the structure diameter was set to 48" because there was no 54" in the drop down list. I used the part builder to add 54" to the list and then went into my parts list and added that part size of 54" and associated it with my 54" style.
But I still don't have that 54" size available to me in the drop-down menu under structure properties! I cannot for the life of me figure out how to make this 54" size available to me in the drop down list even after creating it in the parts builder and adding it to my parts list. Any ideas?
I am using Civil3D 2014.
Ah ha! I just got it. Instead of trying to edit the diameter in the structure properties, I just went to "swap part" under the right click menu for the structure and swapped it for the new part I had just created and added to the parts list!