I think I have to leave the answer to someone who is more experiences with ADS layout.
Here is basic information that I have:
If you right click on the component, you can do Component > Edit Component Artwork. There, you can select "fixed" and in the dropdown list below, choose one of the existing layout defintions.
As an alternative, you can also use the "Lumped with Artwork" where you can define the pad size in the component defintion. **broken link removed**
As a third alternative, some components libraries also used "programmed" layout with the AEL macro language.