Place your cursor over Reference Data and left click on the sub-option Incident Size Classes.
Find the incident size class that you would like to delete and left click on the Delete button 
on that line.
A dialog box will appear for you to choose either 
to delete the record or 
to not delete.
If the item you are trying to delete is associated with any of the data in the database IQSweb will not allow you to delete the data.