Hi @Pigeon let's talk bears! First, if you haven't already, I would highly recommend reading this article on backpacking in bear country and this article on how to choose and use bear canisters. You'll find really great info in both of these how-to's.
Bear canisters are rather bulky, and since you should put them in the middle of your pack, they displace a fair amount of space. So with that in mind, I recommend sizing up your pack. You didn't mention how many nights you are planning on spending out there, but I would recommend a minimum of a 65 liter pack. Of course, how many nights, how many people are going with, size of your other gear, etc... all can influence this decision.
Check out How to Choose a Backpack as well. I personally use a 75 liter pack for all my backpacking adventures. If you want specific recommendations or need any more info, let us know.
Have fun out there!
@Pigeon Thanks for reaching out!
@REI-EricP gave you a lot of really good resources that should help you make the decision. As someone who lives and recreates in Alaska, basically we are always in bear country. If you have experience backpacking and are used to how much space you need for your kit, I would recommend adding 10-15 liters of space to take the bear canister into consideration.
As an example, typically I used a 40 liter pack for a one-to-two night backpacking trip when I would go backpacking in areas where bears were not a concern. When I'm carrying a bear canister and backpacking for one-to-two nights I now use a 55 liter backpack to accommodate the canister and my base backpacking kit.
Although it is recommended to put the bear canister inside your backpack, I have seen other backpackers strap their canister to the outside of their backpack (on the back, or under the floating lid at the top). Neither is ideal in terms of comfort and stability, but if you are taking a trip to bear country (and do not typically need the extra room) it could save you from having to purchase a new backpack for just a single trip. Additionally, you may be able to rent a bigger backpack if that option is available in your area.
Where are you thinking about backpacking? How many days and nights are you planning on for your trip? We may also have some folks in the community who could help out with specific advice.
Hope this helps!