Upvote:2 

- (CGSize)collectionView:(UICollectionView *)collectionView
                  layout:(UICollectionViewLayout*)collectionViewLayout
  sizeForItemAtIndexPath:(NSIndexPath *)indexPath
{
    CGRect screenRect = [[UIScreen mainScreen] bounds];
    CGFloat screenWidth = screenRect.size.width;
    float cellWidth = screenWidth / 2.0;  //put number of columns you require
    CGSize size = CGSizeMake(cellWidth, cellWidth);

    return size;
}

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