Use this method to set custom cell height width.

Make sure to add this protocols

UICollectionViewDelegate

UICollectionViewDataSource

UICollectionViewDelegateFlowLayout

If you are using swift 5 or xcode 11 and later you need to set Estimate Size to none using storyboard in order to make it work properly. If you will not set that than below code will not work as expected.

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Swift 4 or Later

extension YourViewController: UICollectionViewDelegate {
    //Write Delegate Code Here
}

extension YourViewController: UICollectionViewDataSource {
    //Write DataSource Code Here
}

extension YourViewController: UICollectionViewDelegateFlowLayout {
    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
        return CGSize(width: screenWidth, height: screenWidth)
    }
}

Objective-C

@interface YourViewController : UIViewController<UICollectionViewDelegate,UICollectionViewDataSource,UICollectionViewDelegateFlowLayout>

- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath
{
    return CGSizeMake(CGRectGetWidth(collectionView.frame), (CGRectGetHeight(collectionView.frame)));
}