iOS — how do you control the size of a modal view controller?

Question

I am presenting a modal view controller. If it matters, it is scrolling up from the bottom. How can I control what portion of the screen it occupies?

EDIT: I have

Answer 1

I would add to @dsaw's answer that the superview of the modal view does not seem to rotate its coordinate system in landscape mode. Here is the code that I used in my own app:

MyViewController* modalVC = [[MyViewController alloc] init];        
modalVC.modalPresentationStyle = UIModalPresentationFormSheet;
[self presentModalViewController:modalVC animated:NO];
CGRect r = CGRectMake(self.view.bounds.size.width/2 - 236,
                      self.view.bounds.size.height/2 - 130,
                      472, 260);
r = [self.view convertRect:r toView:modalVC.view.superview.superview];
modalVC.view.superview.frame = r;

While the superview may not rotate itself with the iPad, it does seem to do the right thing and keep the modal view centered if I rotate the iPad after showing the modal view.