方法一；

• (CGSize) getAttributeSizeWithText:(NSString *)text fontSize:(int)fontSize
  {
  CGSize size=[text sizeWithAttributes:@{NSFontAttributeName: [UIFont systemFontOfSize:fontSize]}];
  if (isIos7Later) {
  size=[text sizeWithAttributes:@{NSFontAttributeName: [UIFont systemFontOfSize:fontSize]}];
  }else{
  NSAttributedString *attributeSting = [[NSAttributedString alloc] initWithString:text attributes:@{NSFontAttributeName: [UIFont systemFontOfSize:fontSize]}];
  size = [attributeSting size];
  }
  return size;
  }

方法二，直接sizetofit方法，会自动换行，然后其frame就是对应的实际大小
方法三

import "Util.h"

import <CommonCrypto/CommonDigest.h>

@implementation Util

• (CGSize)sizeWithString:(NSString *)string font:(UIFont *)font constraintSize:(CGSize)constraintSize
  {
  CGSize stringSize = CGSizeZero;

  NSDictionary *attributes = @{NSFontAttributeName:font};
  NSInteger options = NSStringDrawingUsesFontLeading | NSStringDrawingTruncatesLastVisibleLine | NSStringDrawingUsesLineFragmentOrigin;
  CGRect stringRect = [string boundingRectWithSize:constraintSize options:options attributes:attributes context:NULL];
  stringSize = stringRect.size;

  return stringSize;
  }

来自：http://www.cocoachina.com/bbs/read.php?tid=456568