Yes, the specs do say 26x2.0 ONLY (guess you can't use a 26x1.5, lol).

I think you could actually fit much larger than this, since I still have plenty of clearance with 26x2.0 tires and 60mm fenders. I'm using a ~3/4" spacer below the fork crown now with the fenders set to 2" tire.

Even though the Civia Loring fork has 12mm more atc distance than the stock fork, it may have less tire clearance diameter-wise. I can't say for sure since I don't have a Salsa Caseroll fork to measure, but I suspect they're very close.

I measured carefully for fit. The Loring should take up to a 700x32 wheel+tire with fender, which is coincidentally the specified tire size for a Salsa Casseroll. It doesn't look like this bike will actually take much larger tires in stock form.

My 700x28 fits the Civia Loring with a centimeter to spare for fender and clearance.