Selects allows a single option from a predefined list. It is a good scalability solution if you have more than 3-4 options to select from. In Sitefinity selects are used primarily as choices fields in forms, a way to switch content languages or to switch sorting options.
Large selects are used only as fields in forms. Their width is 75% of the overall form width. If the field can be left empty add a not selected option to the list. If selecting an option is mandatory specify a default option to be preselected.
Small select is used only in the filter’s sidebar of the List view screens to switch sorting options or languages of the content, as well as in the Compare translations sidebar in the Edit screen to switch available translations. A default option must always be preselected.