Поместите это в ViewDidAppear:
NSLayoutConstraint *heightConstraint =
[NSLayoutConstraint constraintWithItem: self.view
attribute: NSLayoutAttributeHeight
relatedBy: NSLayoutRelationEqual
toItem: nil
attribute: NSLayoutAttributeNotAnAttribute
multiplier: 0.0
constant: 300];
[self.view addConstraint: heightConstraint];
Работает в iOS 8.1