Upvote:3
In theory you're supposed to be told what applies at ticketing:
Airlines and travel agents must also specify at the time of ticketing the carry-on allowance and/or charges
In practice it would not surprise me to find that that does not include actual dimensions as the airlines like to retain the ability to change these without warning, as per your linked article.
If the actual dimensions are not specified (only the number and/or charges) I would suggest adhering to the more restrictive of the flights to avoid unnecessary hassle at the airport because you don't have a fallback position of showing your baggage dimensions in writing, only your allowance.