UILineBreakModeWordWrap is deprecated

You need to use NSLineBreakByWordWrapping in iOS 6

For your code try this:

NSString *string = @"bla";

CGSize s = [string sizeWithFont:[UIFont systemFontOfSize:20]
              constrainedToSize:CGSizeMake(self.view.bounds.size.width - 40, CGFLOAT_MAX) // - 40 For cell padding
                  lineBreakMode:NSLineBreakByWordWrapping];

an example on a label would be:

[label setLineBreakMode:NSLineBreakByWordWrapping];

Instead of

label.lineBreakMode = UILineBreakModeWordWrap;