How do I define the size of a CollectionView on rotate

Heres my 2 cents – because your item sizes are static why don’t you set the item size on the collectionViewLayout?

It will be quicker to do this rather than expecting the collection view to call its delegate method for every cell.

viewWillLayoutSubviews can be used for detection of rotation on view controllers.
invalidateLayout can be used on a collection view layout to force it to prepare the layout again causing it to position the elements with the new sizes in this case.

- (void)viewWillLayoutSubviews;
{
    [super viewWillLayoutSubviews];
    UICollectionViewFlowLayout *flowLayout = (id)self.firstCollectionView.collectionViewLayout;

    if (UIInterfaceOrientationIsLandscape(UIApplication.sharedApplication.statusBarOrientation)) {
        flowLayout.itemSize = CGSizeMake(170.f, 170.f);
    } else {
        flowLayout.itemSize = CGSizeMake(192.f, 192.f);
    }

    [flowLayout invalidateLayout]; //force the elements to get laid out again with the new size
}

edit: updated with Swift2.0 example

override func viewWillLayoutSubviews() {
  super.viewWillLayoutSubviews()

  guard let flowLayout = collectionView.collectionViewLayout as? UICollectionViewFlowLayout else {
    return
  }

  if UIInterfaceOrientationIsLandscape(UIApplication.sharedApplication().statusBarOrientation) {
    flowLayout.itemSize = CGSize(width: 170, height: 170)
  } else {
    flowLayout.itemSize = CGSize(width: 192, height: 192)
  }

  flowLayout.invalidateLayout()
}