If I recall correctly, you are in the UK. Carradry panniers may make sense too. They are bigger than Ortliebs by a small bit, waterproof, etc. I bought some years ago, I used them on one tour in 2019 in back. Photo below:
The blue bag on top is a drybag that was not specifically sold for cycling.
Like Ortliebs they are waterproof. But for you they might be cheaper since the company that makes them is in UK. After Brexit, shipping costs to USA from UK went way up, so a lot of things made in UK are more costly for us in USA now. I used to buy a lot of stuff from the UK.
https://www.sjscycles.co.uk/luggage/...-grey-58-litre
I usually use my Ortliebs, for one thing I have the 31 liter Rack Pack that mates with my back rollers really well. I like the Ortlieb hooks for hanging on the panniers much better than the Carradry too.
My Ortliebs in the photo below.
On the Pacific Coast route, you will have opportunities to buy food every few days, so you do not need to carry a lot. Keep that in mind. I often tour off the beaten path and often am carrying over a week of food, that takes more volume capacity.
Each of my tours, I ask myself should I bring my Carradry panniers or Ortliebs? Both have advatages and disadvantages.
I think the Ortliebs are better made, but that also means they are heavier because the fabric is thicker, etc. Backrollers are rated for 40 liters. The Carradry are now rated at 48.
I initially bought my Carradry rear panniers for a trip where I needed a lot of capacity. At that time Carradry claimed they held 58 liters. They arrived in the mail and my first thought was, these are bigger than Ortliebs, but much smaller than 58. Now they are rated at 48 liters.
One other disadvantage of the Carradry, if they are not full, the lid on top might not fit as well as when full. But my Ortlieb Backrollers, if they are not full, I can roll the top one or two more turns, they work well.
If you are trying to figure out the volume of your stuff, put it all in a box. If the box is not full, move things around in the box so there is a reasonably flat surface on the top of your stuff in that box. Then measure width, length and the height of your stuff in the box. Calculate volume. If you have a separate handlebar bag, keep that stuff out because that is not in your panniers.