How do I get the largest font size in a UILabel of a given size?

Hi. I have a UILabel of a fixed size. This size cannot be changed, as the interface is all set up to have that label there at that particular size.

I'm trying to get the largest font size for any font I might put in that label, that will fit all of the text inside that label.

The label can have up to four lines, with the text contents being centred horizontally and vertically.

I've seen a bunch of suggestions about using boundingRectWithSize:options:attributes:context but that always returns the maxium I put in, i.e. 50. Here's a snippet of my code:

  theLabel.frame = labelRect;
  theLabel.lineBreakMode = NSLineBreakByWordWrapping;

  // Start at the largest size
  CGFloat fontSize = 50.0;

  CGSize constraintSize = CGSizeMake(theLabel.frame.size.width, theLabel.frame.size.height);  // This is the size of my UILabel
  NSStringDrawingContext *context = [NSStringDrawingContext new];
  context.minimumScaleFactor = 0.5;
  do {
    // Set the font to the current size
    theLabel.font = [UIFont fontWithName:theLabel.font.fontName size:fontSize];

    // Find label size for current font size
    CGRect textRect = [[theLabel text] boundingRectWithSize:constraintSize
    options:NSStringDrawingUsesLineFragmentOrigin|NSStringDrawingUsesFontLeading
    attributes:@{NSFontAttributeName:theLabel.font}
    context:context];
    CGSize labelSize = textRect.size;
    NSLog(@"labelSize.height = %.2f, theLabel.frame.size.height = %.2f", labelSize.height, theLabel.frame.size.height);

    // Done, if created label is within target size
    if(ceilf(labelSize.height) <= ceilf(theLabel.frame.size.height))
      break;

    // Otherwise, redice the font size and try again
    fontSize -= 1.0;
  } while(fontSize > 5.0);  // Minimum size = 5.0

NSLog(@"new font size = %.2f", fontSize);
[theLabel setFont:[UIFont fontWithName:theLabel.font.fontName size:fontSize]];

The output is this:

labelSize.height = 0.00, theLabel.frame.size.height = 149.00

new font size = 50.00

What am I doing wrong? Why does labelSize.height always equal zero?