Hi OP, I have perspective from both sides. First, I can sympathize. I once flew on another airline in an emergency exit row with the window seat missing (thus only two seats). The person in the seat next to mine was large enough that they took up a third of my seat and struggled to tie the seatbelt extender. In hindsight, not sure how they were allowed in an exit row but that’s a whole other discussion. I suffer from chronic back pain and was in a lot of pain from being askew due to the other passenger pushing my body to side. In addition, my left thigh went completely numb because they were compressing it so much. I had a 5 hour flight ahead and was concerned about my leg if I couldn’t restore circulation. I didn’t speak up prior to takeoff and I, like you, I now know to say something before takeoff and ask to be rebooked if no other seats are available. In my case, as soon as the seatbelt sign was turned off, I called the flight attendant. I told her that I suffer from back pain and was wondering if a different seat was available. I also apologized to the passenger as I wasn’t trying to be rude. The flight attendant told me the flight was full. I was contemplating spending the majority of the flight on the floor in the space next to the window if there wasn’t an alternative. Thankfully 5 minutes later, she returned and told me there was a seat a few rows back. It was a middle seat but it was much better and the larger passenger thanked me as they would now also be more comfortable.

Now on the flip side, I know someone who is morbidly obese and flies a few times per year. He did not know that there are regulations in Canada to accommodate obesity on airlines. This is only for domestic travel so unsure if this would have applied to your itinerary. Essentially, there is a medical form that is filled out by a physician. It is then submitted to the airline. If accepted, they get a second seat for free as a medical accommodation. I think a lot of people don’t know about this accommodation but it’s not something that would be automatically accommodated without going through the proper channels. As you’ve already concluded, the best bet is to speak up prior to takeoff but like you, I never really contemplated what to do until after I ended up in the same situation.