From ReactOS

Windows DLG and Dialog Structure

typedef struct _DLG
{
/* 000 */ DLGPROC  lpfnDlg;
/* 004 */ DWORD    flags;        // 1 == Ansi
/* 008 */ INT      cyChar;
/* 00c */ INT      cxChar;
/* 010 */ HWND     hwndFocusSave;
/* 014 */ UINT     fEnd:1;
          UINT     fDisable:1;
/* 018 */ LONG_PTR result;
/* 01c */ HANDLE   hData;
/* 020 */ HFONT    hUserFont
} DLG, *PDLG; // Size: 0x24

// Window Extra data container.
typedef struct _DIALOG
{
  WND     wnd;         // Index offset:
  LRESULT resultWP;    // 00 DWLP_MSGRESULT
  PDLG    pdlg;        // 04 DWLP_DLGPROC
  DWORD   unused;      // 08 DWLP_USER
  DWORD   reserved[5]; //
} DIALOG, *PDIALOG; // w2k size 0xb8 = sizeof(WND) + DLGWINDOWEXTRA 30 bytes round up to 32

References

• Windows Symbol files, userkdx.dll, !dso