Rachael Van Dolah wrote an answer about DAKINE Point Wet/Dry Backpack - 1800cu in on July 22, 2014
Just to clarify, this pack is not a dry bag but rather is referred to as a dry/wet pack as it comes with a back pocket that has space for a rashguard or wetsuit. If you are in need for a waterproof pack you may want to look into alternative styles. As for compressibility, this will get quite thin. The pack dimensions are 19 x 12 x 9", but will flatten to about 19 x 12 x 1.5 to 2 inches when empty.