Formed in Carboniferous/Permian limestone, the main Sơn Đoòng cave passage is the largest known cave passage in the world by volume – 3.84×10⁷ m³ (1.36×10⁹ cu ft), according to BCRA expedition leader Howard Limbert. It is more than 5 kilometres (3.1 mi) long, 200 metres (660 ft) high and 150 metres (490 ft) wide.
So that'd be nearly triple the volume of the Everett Factory. Though the cave has two holes in its roof, and I don't know exactly how you define "room" here.
They had to change their venting and airflow system for that building after it formed a cloud and rained inside. When your room can have weather systems, I feel you've entered a whole new category of 'room' by definition.
Factories will win this hands down, especially when you're building large/complex items. It looks like the distinction might be "single building" vs "complex or buildings", but VW's Wolfsburg plant is 70 million square feet. The largest plant I've been to isn't on that list, but it's still over a half mile wide - all under a single roof.
It's actually kind of amazing to see a building with multiple assembly lines of wide body airplanes. The tour is well worth the drive to Everett if you are ever in Seattle.
I got lucky at a conference. They got us a VIP tour of the Boeing Everett factory, which walked on the assembly floor. It was a phenomenal experience. The sheer scale of the operation, the size of the planes, and the detail work was astounding.
I went looking but couldn't find a reference. US Air Force Plant 4 in Fort Worth (where the F-35 is assembled) was at one point the longest length building without internal support columns. I've been told that there is a twin building somewhere else, but the one in Texas is 25 feet longer. I just can't find a source with the number!