Create multiple types... i.e., in Family Editor, click the Family Types... button on the design bar. Then under Types, click New.
Assuming you have built your family parametrically, you can modify the parameters of each type independent, thereby creating multiple types/sizes within the same family file.
If they are truly different types of slot diffusers (i.e., one with a rectangular duct connection, and the other with a round duct connection), your best bet is to create those as different families, and saving with different file names.
Martin Schmid
Product Line Manager
Mechanical Detailing and Electrical Design
Architecture, Engineering, and Construction
Autodesk, Inc.