Hi Terry and mmara,
The section that Peter is referring to can be found by opening your listing and clicking the "Custom Item Specifics" link that is present just below the category field. There are two fields for each custom item specific there, which are labeled "Detail Name" and "Value".
The error message you are referring to implies that one of your entries has a "Detail Name" that is more than 30 characters which, as Peter mentioned, includes spaces and special characters. I believe the "Detail Name" field supports up to 30 characters and the "Value" field supports up to 20 characters.
If you have any additional questions or concerns, please feel free to contact our customer support team using the web form on the following page:
http://www.auctiva.com/help/requesthelp.aspx-Mike