They usually have the info under the specs tab. I have usually had great luck with rei and their info on packs. Take another look I bet you just overlooked it. Good luck on your trip. Also, I have heard of guys shipping their stuff ups to a friend, hotel, etc. It may be cheaper and safer.I have shopped several bags on REI and all I have had to do was click on the specs tab and a plethora of information was made available to me. Not sure where you're looking, but I have never had an issue finding capacity on REI.
If you buy the REI duffle I linked to, I can all but garantee your pack will fit. My pack is rated as 2000 cubic inches larger than yours and fits in it even when full.my pack is a kuiu 5200 and having trouble getting it to fit into my sons air force duffle bag. also have full day pack and full riffle case. don't know what not to bring. bags on rei don't tell you what size they are. how dumb is that!
Best use Backpacking, Travel
Dimensions 32 x 17 x 12 inches
Weight 19.7 ounces