minimumFontSize property of the UILabel is deprecated from iOS 6.0 onwards.
An Alternative to the minimumFontSize is minimumScaleFactor. If you assign minimumFontSize/defaultFontSize to minimumScaleFactor, it works in the same way as minimumFontSize.
The Code is as follows – For Example the font size is 30.0 and if you want the minimum font size to be 12.0
YOURLABEL.font= [UIFont fontWithName:@"FONT_NAME" size:30.0];
[YOURLABEL setMinimumScaleFactor:12.0/[UIFont labelFontSize]];