Redimensionar Tform de acordo com o tamanho da string em um Tlabel.

object frmMessageBox: TfrmMessageBox
  Left = 0
  Top = 0
  BorderStyle = bsSingle
  Caption = 'frmMessageBox'
  ClientHeight = 107
  ClientWidth = 377
  Color = clBtnFace
  Font.Charset = DEFAULT_CHARSET
  Font.Color = clWindowText
  Font.Height = -11
  Font.Name = 'Tahoma'
  Font.Style = []
  OldCreateOrder = False
  OnClose = FormClose
  OnCreate = FormCreate
  PixelsPerInch = 96
  TextHeight = 13
  object lblTitle: TLabel
    AlignWithMargins = True
    Left = 10
    Top = 0
    Width = 357
    Height = 31
    Margins.Left = 10
    Margins.Top = 0
    Margins.Right = 10
    Margins.Bottom = 0
    Align = alTop
    AutoSize = False
    Caption = 'TITULO'
    Font.Charset = DEFAULT_CHARSET
    Font.Color = clBlue
    Font.Height = -21
    Font.Name = 'Tahoma'
    Font.Style = [fsBold]
    ParentFont = False
    Layout = tlCenter
    ExplicitLeft = 0
    ExplicitWidth = 367
  end
  object lblMessage: TLabel
    AlignWithMargins = True
    Left = 10
    Top = 31
    Width = 357
    Height = 25
    Margins.Left = 10
    Margins.Top = 0
    Margins.Right = 10
    Margins.Bottom = 10
    Align = alClient
    WordWrap = True
    ExplicitLeft = 0
    ExplicitWidth = 3
    ExplicitHeight = 13
  end
  object Panel1: TPanel
    Left = 0
    Top = 66
    Width = 377
    Height = 41
    Align = alBottom
    BevelEdges = [beTop]
    BevelKind = bkFlat
    BevelOuter = bvNone
    TabOrder = 0
    object BOK: TButton
      Left = 6
      Top = 6
      Width = 75
      Height = 25
      Caption = 'OK'
      ModalResult = 1
      TabOrder = 0
    end
  end
end

unit Unit7;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.ExtCtrls, Vcl.StdCtrls;

type
  TfrmMessageBox = class(TForm)
    lblTitle: TLabel;
    lblMessage: TLabel;
    Panel1: TPanel;
    BOK: TButton;
    procedure FormCreate(Sender: TObject);
    procedure FormClose(Sender: TObject; var Action: TCloseAction);
  private
    { Private declarations }
    FLabelHeight: Integer;
    function CalcRecLabel(ALabel: TLabel; AWidth: Integer): Integer;
  public
    { Public declarations }
    function ShowModal(AMessage: String): Integer; overload;
  end;

var
  frmMessageBox: TfrmMessageBox;

implementation

{$R *.dfm}

function TfrmMessageBox.CalcRecLabel(ALabel: TLabel; AWidth: Integer): Integer;
var
  lRect : TRect;
  lText : string;
begin
  lRect.Left := 0;
  lRect.Right := AWidth;
  lRect.Top := 0;
  lRect.Bottom := 0;

  lText := ALabel.Caption;

  ALabel.Canvas.TextRect(
            lRect,                    //será modificado apra preencher a dimensão do texto
            lText,                    //não é modificado, a menos que você use o flag "tfModifyingString"
            [tfCalcRect, tfWordBreak] //o flag para indicar "calcule a dimensão do texto com as quebras de linha"
          );
  Assert( lRect.Top = 0 );
  //ALabel.Height := lRect.Bottom; //para teste apenas
  Result := lRect.Bottom;
end;



procedure TfrmMessageBox.FormClose(Sender: TObject; var Action: TCloseAction);
begin
  Action:= TCloseAction.caFree;
  frmMessageBox:= nil;
end;


procedure TfrmMessageBox.FormCreate(Sender: TObject);
begin
  FLabelHeight:= lblMessage.Height;
end;


function TfrmMessageBox.ShowModal(AMessage: String): Integer;
var
  h: Integer;
begin
  DisableAlign;

  lblMessage.Caption:= AMessage;
  Application.ProcessMessages;
  lblMessage.Height:= CalcRecLabel(lblMessage, Self.Width);

  Self.Height:= Self.Height + (lblMessage.Height - FLabelHeight);
  FLabelHeight:= lblMessage.Height;

  EnableAlign;

  Result:= inherited ShowModal();

end;
end.

var
  msg: string;
begin
  if not Assigned(frmMessageBox) then frmMessageBox:= TfrmMessageBox.Create(Self);
  msg:= '<Escreva um texto grande aqui>';
  frmMessageBox.ShowModal(msg);
end;