4 years ago or so I bought an Osprey Viper 9, a mt bike pack that was the smallest pack they made that held their biggest water bladder, 3 liters. About 10" wide 16-17" long, pretty flat, and about 500 cubic inches of storage. Has a decent stuff compartment with straps for cinching down a rolled up jacket or bulky item.
https://www.osprey.com/us/en/category/hydration/cycling/Doesn't look like they still carry my exact model, but it would be between the Siskin 12 and Siskin 8. Mine still has more room than I need, but the pack is small enough to not get in the way. I use it for just about everything but overnight trips.
