Because bands are inherited from Panels, they change their size in the same way.  The size of the Band can be automatically changed depending on the size of components positioned on the band.

 

CanGrow Property

It should be noted that most types of band can only automatically change their height - the exception is cross-bands which change their width. For example, if there is a component on the band which crosses the lower boundary and you set the CanGrow property of the band to true, the band height will be automatically increased until the entire component is contained within the band:

 

 

CanShrink Property

Similarly if there is free space between the boundary of a band and the lower border of the tallest component that it contains and you set the CanShrink property to true, the height of the band will automatically be reduced until it matches the lowest point of the lowest contained component: