Multi-line UIlLabel in full-width UICollectionViewCell.

CollectionViewLabelCell

//In your UIViewController (containing the UICollectionView).
//In @interface
  @property (nonatomic, strong) MyCollectionViewCellSubclass *prototypeCell;

//In viewDidLoad
  //Register your subclassed cell, and load your prototype cell.
    UINib *cellNib = [UINib nibWithNibName:@"MyCollectionViewCellSubclass" bundle:nil];
    [collectionView registerNib:cellNib forCellWithReuseIdentifier:@"MyCollectionViewCellSubclassID"];
    self.prototypeCell = [cellNib instantiateWithOwner:nil options:nil][0];
  
//Add this delegate method.
- (CGSize)collectionView:(UICollectionView *)collectionView
                  layout:(UICollectionViewLayout *)collectionViewLayout
  sizeForItemAtIndexPath:(NSIndexPath *)indexPath {
    //Populate your cell the same way you do in cellForItemAtIndexPath:.
    //Here I have a method to do that:
    [self populateCell:self.prototypeCell atIndexPath:indexPath];
 
    //This line forces the label to it's max width.
    self.prototypeCell.label.preferredMaxLayoutWidth = self.prototypeCell.label.frame.size.width;
    CGSize size = [self.prototypeCell.contentView systemLayoutSizeFittingSize:UILayoutFittingCompressedSize];
    //Make sure the cell fills the horizontal space.
    size.width = CGRectGetWidth(collectionView.frame);
}