Having lived in the rainy Willamette Valley for 6+ years, I’d say you’d be happier and be financially ahead with the indoor. I know that I don’t want to have to deal with trying to care for wet tack every day and, if you allowed me to choose to ride or not depending on the rain, I wouldn’t ride nearly enough during our very long rainy season.
Also, engineering drainage here is non-trivial. I have watched people do it wrong and spend tens of thousands re-doing an arena or paddock drainage because they didn’t truly appreciate what it takes to build an all-weather surface on clay soil with this much rain.
As to size, I have been quite happy in 80’x160’+ arenas I have ridden in. I have also lived in the Northeast where very closed in 60’x120’ were the normal dimensions. Anything wider than 60’ feels good. The length of the trusses and the cost of going wide will most likely determine how wide you can build. As someone else said, 80’ wide feels like the minimum I’d want for jumping. It’s plenty wide for flat work and, of course, for a dressage court. I’d build so as to leave the walls as visually open as I could. Lots of folks here leave two sides that are downwind from the prevailing winds open. Letting horses see out of an indoor, even when the space is smaller helps them stay willing to go forward.
As to dust, that’s not such a problem with the side(s) open. A favorite barn design here is a high pole barn/arena with the stalls going down the long side, built as a lean-to, and the paddocks with runs off of those. IMO, the really smart money builds some eaves out from those stalls so as to create a 12’x12’ covered “porch” for the paddocks.
Lots of people build DIY (and pretty good) PVC-pipe based sprinkler systems and that helps with the dust. I have seen some that run along the rail and some that run along the cross members up above. Just design yours with the realities of your water pressure in mind. Nothing is worse that creating a strip of ever-dry sand down the middle. It makes the footing weird and maintenance a little tricky.
I think it does suck a little bit to have to choose between the two, as an indoor does feel confining to a horse in comparison with a larger outdoor. But the cost of building both is so high, and the length of our rainy season is so long, that I still would to the indoor first.
Best of luck in your decision-making!