设置文本字体大小

   CGSize maxSize = CGSizeMake(ScreenW, MAXFLOAT); //设置一个行高上限

    NSDictionary *attribute = @{NSFontAttributeName:self.subtitleLabel.font};

    CGSize subtitleSize = [self.item.subtitle boundingRectWithSize:maxSize options:NSStringDrawingUsesLineFragmentOrigin attributes:attribute context:nil].size;