Thanks for the suggestion Peter. I like the last line “just go for it.” I’ll go back to REI and see if they will exchange mine for a 40L That seems like great advice. The lighter the better especially since I turn 68 on day one of Camino!!! My last CF was almost 10 years ago.
Rather than settling on a specific size, you might try doing a couple of quick checks.
First, do an initial estimate of the pack volume needed to support someone of your weight and height. My quick rule of thumb is to confirm your 'healthy' weight for your height. A BMI of 25 is a good measure, but I haven't been that light for far too many years. Take that weight (in kg), halve that and that is the volume that I would recommend you start with when shopping for a
summer camino. Add 25% to that for the extra volume of warmer clothing you might need.
Second, follow the advice from
@Peterexpatkiwi, collect everything you plan to take, and take that with you when you shop. How you pack it at this stage is unimportant. What is important is that you take everything, even the small items that one regularly sees people suggesting 'this weighs nothing'!
At this point, if what you are trying to pack is significantly more than can be contained in the pack volume suggested at step one, it might be time to seek advice from the forum about what you are packing. There are plenty of people here that will help you with that. You need to figure out who the ultralight-weight extremists are, and whether you are willing to follow their advice. Otherwise, there are plenty of quite ordinary people who offer good advice on this.
Once you have everything in a suitably sized pack and it as been fitted properly, test for comfort. Wear it around they store, bounce up and down, twist around, do laps, whatever it takes within the boundaries of doing this in a store to check that it is comfortable. If you are new to this, I recommend doing this with several similar sized packs from different manufacturers, even if it means going to several outfitters. You might choose the first pack you tried, but you will have a better understanding that this was the best choice once you have compared it to others.
Going back to your 55li pack. Reverse engineering my rule of thumb, for an autumn camino, these would be the starting points for someone about 1.87m tall or weighing about 88kg. So I think it would be too big for most people on the camino. However, if it is a comfortable pack and doesn't weigh too much, you might choose not to buy another pack. What you will need is the discipline to not carry more than you really need. Packs have an almost magical ability to have their load expand to the load they can carry, and that would not be a good outcome!