How can I get the actual size of a UIVIewController view open modally?

Question

I'm opening a view controller modally and on the ViewDidLoad of the UIViewController that has been opened modally I need to know its width and height. This became a problem when I tested on a iPad 12.9 inches. UIViewControllers open modally will not occupy the full screen height or width, which if fine, but I need to know the size of this view.

This is the way I am opening a UIViewController modally:

viewController.modalTransitionStyle = UIModalTransitionStyleCoverVertical;
[self presentViewController:viewController animated:YES completion:NULL];