Buttons and text links are one of the primary ways users interact with a site. It is critical that size, heirarchy, and language are clear. This ensures that users can use sites without confusion or frustration.
Use the large button when there is a single primary action
Use the regular/default button size when there is a single primary action
Use the small button size to distinguish hierarchy between multiple actions on a single page
Use the x-small button size for lowest priority actions