谢谢大家的帮助，这是我尝试的代码，这是为我工作

   UILabel *instructions = [[UILabel alloc]initWithFrame:CGRectMake(10, 225, 300, 180)];
   NSString *text = @"First take clear picture and then try to zoom in to fit the ";
   instructions.text = text;
   instructions.textAlignment = UITextAlignmentCenter;
   instructions.lineBreakMode = NSLineBreakByWordWrapping;
   [instructions setTextColor:[UIColor grayColor]];

   CGSize expectedLabelSize = [text sizeWithFont:instructions.font 
                                constrainedToSize:instructions.frame.size
                                    lineBreakMode:UILineBreakModeWordWrap];

    CGRect newFrame = instructions.frame;
    newFrame.size.height = expectedLabelSize.height;
    instructions.frame = newFrame;
    instructions.numberOfLines = 0;
    [instructions sizeToFit];
    [self addSubview:instructions];

在iOS 6中，苹果为UILabel添加了一个属性，极大地简化了标签的动态垂直大小调整:preferredmaxlayoutidth。

将此属性与lineBreakMode = nslinebreakbywordwrapped和sizeToFit方法结合使用，可以轻松地调整UILabel实例的大小，以容纳整个文本的高度。

引用iOS文档:

preferredMaxLayoutWidth 多行标签的首选最大宽度(以点为单位)。 讨论 当对标签应用布局约束时，此属性将影响标签的大小。在布局过程中，如果文本扩展到超出此属性指定的宽度，则附加文本将流动到一个或多个新行，从而增加标签的高度。

一个示例:

...
UILabel *status = [[UILabel alloc] init];
status.lineBreakMode = NSLineBreakByWordWrapping;
status.numberOfLines = 5; // limits to 5 lines; use 0 for unlimited.

[self addSubview:status]; // self here is the parent view

status.preferredMaxLayoutWidth = self.frame.size.width; // assumes the parent view has its frame already set.

status.text = @"Some quite lengthy message may go here…";
[status sizeToFit];
[status setNeedsDisplay];
...

最后，它成功了。谢谢大家。

我没有让它工作，因为我试图在heightForRowAtIndexPath方法中调整标签的大小:

- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath

和(是的，我很傻)，我在cellForRowAtIndexPath方法中调整标签的默认大小-我忽略了我之前写的代码:

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath

你也可以把它作为一种方法。@Pyjamasam是非常正确的，所以我只是在创建它的方法。它可能对其他人有帮助

-(CGRect)setDynamicHeightForLabel:(UILabel*)_lbl andMaxWidth:(float)_width{
    CGSize maximumLabelSize = CGSizeMake(_width, FLT_MAX);

    CGSize expectedLabelSize = [_lbl.text sizeWithFont:_lbl.font constrainedToSize:maximumLabelSize lineBreakMode:_lbl.lineBreakMode];

    //adjust the label the the new height.
    CGRect newFrame = _lbl.frame;
    newFrame.size.height = expectedLabelSize.height;
    return newFrame;
}

就像这样

label.frame = [self setDynamicHeightForLabel:label andMaxWidth:300.0];

NSString *str = @"Please enter your text......";
CGSize lblSize = [str sizeWithFont:[UIFont systemFontOfSize:15] constrainedToSize: CGSizeMake(200.0f, 600.0f) lineBreakMode: NSLineBreakByWordWrapping];

UILabel *label = [[UILabel alloc]init];
label.frame = CGRectMake(60, 20, 200, lblSize.height);
label.numberOfLines = 0;
label.lineBreakMode = NSLineBreakByWordWrapping;
label.font = [UIFont systemFontOfSize:15];
label.text = str;
label.backgroundColor = [UIColor clearColor];
[label sizeToFit];
[self.view addSubview:label];

此方法适用于iOS 6和7

- (float)heightForLabelSize:(CGSize)maximumLabelSize  Font:(UIFont *)font String:(NSString*)string {

if ([[[UIDevice currentDevice] systemVersion] floatValue] >= 7) {
    NSDictionary *stringAttributes = [NSDictionary dictionaryWithObject:font forKey: NSFontAttributeName];

    CGSize adjustedLabelSize = [string maximumLabelSize
                                                                  options:NSStringDrawingTruncatesLastVisibleLine|NSStringDrawingUsesLineFragmentOrigin
                                                               attributes:stringAttributes context:nil].size;
    return adjustedLabelSize.height;
}
else {
    CGSize adjustedLabelSize = [string sizeWithFont:font constrainedToSize:maximumLabelSize lineBreakMode:NSLineBreakByWordWrapping];

    return adjustedLabelSize.height;
}

}