If you have both a domestic and an international (e.g. transatlantic) leg on your flight, checked luggage allowances are determined by the international leg.
Recently, some US domestic carrier have made stricter rules for carry-on luggage on domestic flights by limiting the size of the bags (especially width). Do similar rules apply for carry-on luggage as for checked luggage, i.e. could you get around these restrictions on the domestic leg if you have a connecting international leg, or would you need to check-in your carry-on on the domestic leg of the flight?