The subject says it all. The website tells me what size Cell I need for a standard W500 (size 1), but the 9-cell battery protrudes from the rear another 1/2-3/4".
Front to back (including battery), the W500 is EXACTLY the same measurement as a 14", non-widescreen T60p with the 9-cell, which fits in a size 3S Cell. It's nearly exactly 11" front of screen to the end of the battery. The 3S cell supposedly is only 10.3" long, but my 9" 14" non-widescreen fits OK in there. I presume the battery is pushing down on the foam suspension a bit, and the velcro doesn't completely overlap, but it's certainly not going anywhere and doesn't feel unsafe.
Width is nearly exactly 14", and height is around 1.375" tall on the W500.
I think I'm going to just try the Size 1 - the depth measurement is identical, so if the fit is like the 3S it should work OK...