UIViewController 属性preferredCont

preferredContentSize The preferred size for the view controller’s view.

Discussion

The value in this property is used primarily when displaying the view controller’s content in a popover but may also be used in other situations. 
Changing the value of this property while the view controller is being displayed in a popover animates the size change; 
however, the change is not animated if you specify a width or height of 0.0.