Sorry for the late reply getting back to people.
The route I'm more specifically asking about is FRA > YYZ. It's a 9 hour flight, and I don't sleep well on planes so I'm looking for the most comfort. If I stay in Economy, and the seat next to me is free, I feel like I'll be fine. If the seat next to me I taken, being a bigger guy, it's going to be uncomfortable for both me and them.
Here are the seatmaps for the flight from both AC and Expert Flyer
I've had a look using the ExperyFlyer flight availability tool, and if I'm reading it right, it seems the bid upgrade to Premium Economy will be a moot point as there are none available. However, again if I'm reading it correctly, there are at least 108 seats still available in Economy.