UIViewController* modalController = [[UIViewController alloc]init];
modalController.modalTransitionStyle = UIModalTransitionStyleCrossDissolve;
modalController.modalPresentationStyle = UIModalPresentationFormSheet;

//Resizing size
CGPoint frameSize = CGPointMake([[UIScreen mainScreen] bounds].size.width*0.95f, [[UIScreen mainScreen] bounds].size.height*0.95f);
CGRect screenRect = [[UIScreen mainScreen] bounds];
CGFloat screenWidth = screenRect.size.width;
CGFloat screenHeight = screenRect.size.height;
if ([[[UIDevice currentDevice]systemVersion] floatValue] >= 8.0 )
{
// Resizing for iOS 8
modalController.preferredContentSize = CGSizeMake(frameSize.x, frameSize.y);
}
else
{
// Resizing for <= iOS 7
modalController.view.superview.frame = CGRectMake((screenWidth - frameSize.x)/2, (screenHeight - frameSize.y)/2, frameSize.x, frameSize.y);
}
UIViewController *vc = [[[[UIApplication sharedApplication] delegate] window] rootViewController];
[vc presentViewController:modalController animated:YES completion:nil];

关于ios 8 7 下的模态窗口大小的控制( Resizing UIModalPresentationFormSheet )