NSString * myText = [NSString stringWithString:@"some text"];
//获取到文本大大小
CGFloat constrainedSize = 265.0f; //其他大小也行
UIFont * myFont = [UIFont fontWithName:@"Arial" size:19]; // UILabel使用的字体
CGSize textSize = [myText sizeWithFont: myFont
                       constrainedToSize:CGSizeMake(constrainedSize, CGFLOAT_MAX)

lineBreakMode:UILineBreakModeWordWrap];

textSize.width

textSize.height

即为UILable的宽和高，

CGRect labelFrame = CGRectMake (0, 0, textSize.width, textSize.height);
UILabel *label = [[UILabel alloc] initWithFrame:labelFrame];

//

UILineBreakModeWordWrap = 0,
   以单词为单位换行，以单位为单位截断。
   UILineBreakModeCharacterWrap,
   以字符为单位换行，以字符为单位截断。
   UILineBreakModeClip,
   以单词为单位换行。以字符为单位截断。
   UILineBreakModeHeadTruncation,
   以单词为单位换行。如果是单行，则开始部分有省略号。如果是多行，则中间有省略号，省略号后面有4个字符。
   UILineBreakModeTailTruncation,
   以单词为单位换行。无论是单行还是多行，都是末尾有省略号。
   UILineBreakModeMiddleTruncation,
   以单词为单位换行。无论是单行还是多行，都是中间有省略号，省略号后面只有2个字符。

//

// Single line, no wrapping. Truncation based on the UILineBreakMode.
- (CGSize)sizeWithFont:(UIFont *)font; // Uses UILineBreakModeWordWrap
- (CGSize)sizeWithFont:(UIFont *)font forWidth:(CGFloat)width lineBreakMode:(UILineBreakMode)lineBreakMode;

// Single line, no wrapping. Truncation based on the UILineBreakMode.
- (CGSize)drawAtPoint:(CGPoint)point withFont:(UIFont *)font; // Uses UILineBreakModeWordWrap
- (CGSize)drawAtPoint:(CGPoint)point forWidth:(CGFloat)width withFont:(UIFont *)font lineBreakMode:(UILineBreakMode)lineBreakMode;

// Wrapping to fit horizontal and vertical size. Text will be wrapped and truncated using the UILineBreakMode. If the height is less than a line of text, it may return
// a vertical size that is bigger than the one passed in.
// If you size your text using the constrainedToSize: methods below, you should draw the text using the drawInRect: methods using the same line break mode for consistency
- (CGSize)sizeWithFont:(UIFont *)font constrainedToSize:(CGSize)size; // Uses UILineBreakModeWordWrap
- (CGSize)sizeWithFont:(UIFont *)font constrainedToSize:(CGSize)size lineBreakMode:(UILineBreakMode)lineBreakMode; // UITextAlignment is not needed to determine size

// Wrapping to fit horizontal and vertical size.
- (CGSize)drawInRect:(CGRect)rect withFont:(UIFont *)font; // Uses UILineBreakModeWordWrap and UITextAlignmentLeft
- (CGSize)drawInRect:(CGRect)rect withFont:(UIFont *)font lineBreakMode:(UILineBreakMode)lineBreakMode; // Uses UITextAlignmentLeft
- (CGSize)drawInRect:(CGRect)rect withFont:(UIFont *)font lineBreakMode:(UILineBreakMode)lineBreakMode alignment:(UITextAlignment)alignment;

// These methods will behave identically to the above single line methods if the string will fit in the specified width in the specified font.
// If not, the font size will be reduced until either the string fits or the minimum font size is reached.  If the minimum font
// size is reached and the string still won't fit, the string will be truncated and drawn at the minimum font size.
// The first two methods are used together, and the actualFontSize returned in the sizeWithFont method should be passed to the drawAtPoint method.
// The last method will do the sizing calculation and drawing in one operation.
- (CGSize)sizeWithFont:(UIFont *)font minFontSize:(CGFloat)minFontSize actualFontSize:(CGFloat *)actualFontSize forWidth:(CGFloat)width lineBreakMode:(UILineBreakMode)lineBreakMode;

- (CGSize)drawAtPoint:(CGPoint)point forWidth:(CGFloat)width withFont:(UIFont *)font fontSize:(CGFloat)fontSize lineBreakMode:(UILineBreakMode)lineBreakMode baselineAdjustment:(UIBaselineAdjustment)baselineAdjustment; // default is UIBaselineAdjustmentAlignBaselines

- (CGSize)drawAtPoint:(CGPoint)point forWidth:(CGFloat)width withFont:(UIFont *)font minFontSize:(CGFloat)minFontSize actualFontSize:(CGFloat *)actualFontSize lineBreakMode:(UILineBreakMode)lineBreakMode baselineAdjustment:(UIBaselineAdjustment)baselineAdjustment; // default is UIBaselineAdjustmentAlignBaselines