Size is represented with the  icon.

Size is used almost all the time when designing a visualization. It can be used to change the size of the Marks or, as with Color, to add more information.

If you don't put any fields on the Size property, clicking on the Size button opens a small menu where you can change the size of the Marks. 

If you put a field on Size, new information is added to the visualization. If you use a Discrete field, each value has a specific size. If you use a Continuous field, Tableau uses a range of size from the minimum to the maximum.

The following screenshot is an example of a Continuous field in Size:

When using a field on Size, a Size legend appears (usually on the right). If you double-click on the Legend, a new menu opens. In this menu, you can choose how the size varies but also the start, end, and size range. Here's the menu:

The next property reacts differently depending on the Mark type.