In a use case diagram, the system boundary is a boundary surrounding the use cases which indicates the system.
Resize a System Boundary shape
Select the shape, and then drag a selection handle.
Rename a System Boundary shape
Select the shape, and then type the name you want.
Move a system boundary and the use cases it contains
Drag a selection rectangle around the System Boundary shape to select it and all the Use Cases, and then drag the shapes to a new location.