The following example demonstrates drawing word wrapped text directly on a TBitBtn's glyph. Example:

procedure TForm .FormCreate(Sender: TObject);
var
  R : TRect;
  N : Integer;
  Buff : array[0..255] of Char;
begin
  with BitBtn  do begin
    Caption := 'A really really long caption';
    Glyph.Canvas.Font := Self.Font;
    Glyph.Width  := Width - 6;
    Glyph.Height := Height - 6;
    R := Bounds(0, 0, Glyph.Width, 0);
    StrPCopy(Buff, Caption);
    Caption := '';
    DrawText(Glyph.Canvas.Handle,
             Buff,
             StrLen(Buff),
             R,
             DT_CENTER or DT_WORDBREAK or DT_CALCRECT);
    OffsetRect(R,
               (Glyph.Width - R.Right) div 2,
               (Glyph.Height - R.Bottom) div 2);
    DrawText(Glyph.Canvas.Handle,
             Buff,
             StrLen(Buff),
             R,
             DT_CENTER or DT_WORDBREAK);
  end;
end;