I just found my venue toady! And it has a big barn that fits up to 199 people. It's in Canby called Postelwaits,http://postlewaits.com/ They can be spendy, but if your date is flexible is can be reasonable. The actual venue looks so amazing in person compared to the pictures, I was blown away. It’s worth a look J
You could try Lucy's Garden. It's in Ridgefiel, WA not far from Portland. Or Pomeroy Farms.
Was going to go with that venue myself because i as well was looking for a rustic barn venue and they were either way out of our budget or didnt have our date. Best of luck!