Thanks to all for your responses.

I'm leaning towards risking it. Wearing the backpack will make the move much easier than having to find a place to cram it into a suitcase. It won't be packed super full and as Airprox mentioned it doesn't look especially big.

The alternative is carrying my TUMI Alpha Bravo Dover backpack at 4.5" x 11.5" x 16", but that depends quite heavily on how much extra room I have to pack the larger one in a suitcase. Further it would be an incovience as I always pack all my essentials in the larger backpack, for which the Dover doesn't have room.