-(void)fitFontSize:(UILabel*)label {
CGFloat actualFontSize;
[label.text sizeWithFont:label.font
minFontSize:label.minimumFontSize
actualFontSize:&actualFontSize
forWidth:label.bounds.size.width
lineBreakMode:label.lineBreakMode];
label.font = [label.font fontWithSize:actualFontSize];
label.adjustsFontSizeToFitWidth = NO;
}

usage: self.keyLabel = [self makeLabel:[UIFont fontWithName:@”AkzidenzGroteskBE-BoldCn” size:45.0f]];

    self.keyLabel.frame = CGRectMake(0, 0, self.cellWidth / 4.0, self.cellHeight);
    self.keyLabel.text = @"C";

    [self fitFontSize:self.keyLabel];
    [self.keyLabel sizeToFit];
    [self.keyLabel makeConstraints:^(MASConstraintMaker *make) {
        make.centerX.equalTo(self.centerX);
        make.centerY.equalTo(self.centerY);
    }];