it's a common misconception, apparently inflicted by the same marketing strategies as those that call a monitor with 20.1" image area a 23-inch monitor.
Iliad outer dimensions are even slightly bigger than A5 (less than 5mm each of the 4 ways). However, if you take the current PDF viewer, and look at the actual area not occupied by the on-screen navigational controls, then it will be just about like A6
in height, and about an inch wider than A6.
Look up the contents howto threads, they will tell you the exact dimensions in mm/inches. Try formatting existing articles with it.
One word of caution to take back something I told about the reformat being not torturous: if the latex source has a lot of display math that is wide enough, it will overflow the right margin and you'll have to manually split it. But as I said, the zoom-in works well enough so that I don't bother to hack into things too deeply.