The power/toughness of the token is set when this spell resolves.

The power and toughness of the Demon token are set when Promise of Power resolves. They’re unaffected if the number of cards in your hand changes later.

If you pay the entwine cost, you draw five cards, then lose five life, then put the Demon token onto the battlefield. The five cards you draw count toward the Demon’s power and toughness.