Well a popular photo backpack is the LowePro Trekker. The LowePro catalog lists it as 17.5x12x6. These are internal dimensions. Just small enough for most airlines. If you are basing your choice of bag on whether you will be able to use it as carry-on luggage, make sure you use external dimensions.
Most catalogs give internal dimensions.
You might need to call the manufacturer for external sizing, or better yet actually measure one for yourself in the store. I have measures 22x8x8, so it should be fine on any of the larger airlines. This is the older, soft sided bag, not the current version with stiffer sides. The regular Trekker mentioned above will not take a 600mm lens. Not all airlines actually make you force your bags in the sizing cube to check the size, but they might.