I purchased this Large Waterproof Kayak Dry Bag years ago, and it was put in the closet and never got used.
In general, because it is rounded in areas, its measurements are 16.5 inches X 27 inches, laying flat.  
For some of the images, I put something inside, a standard backpack, to show what it looks like with something in it.  IT DOES NOT COME WITH THE BACKPACK - for images only.
It has a rubberized interior surface.
Please inspect all images and ask any questions you may have.