EIGHTY-ONE has 9 letters.
ONE HUNDRED has 10 letters.
FIVE HUNDRED AND SEVENTY SIX has 24 letters.
THIRTY-NINE THOUSAND THREE HUNDRED FOUR has 34 letters.
SIXTY-EIGHT THOUSAND NINE HUNDRED AND TWENTY-ONE has 41 letters.
ONE MILLION and ONE BILLION have 10 letters each, making them a sixth root and (in the United States) a ninth root word.
(Dave Morice, “Kickshaws,” Word Ways 30:2 [May 1997], 129-141.)