Both the Ruger and Kahr web sites have this basic info.
If you search Google for <"ruger LCP" "kahr PM9"> then you will find some side-by-side pics of the LCP, PM9, the upcoming P380.
I've had the PM9, the KelTec P3AT 380, the NAA 380 Guardian, and a friend has the LCP. The PM9 is quite a bit larger than all three. The P3AT is tiny, but the P380 is supposed to be even smaller.
Of all of them, the PM9 is 9mm and feels fairly good when firing. Particularly with the 7rd extension magazine, for me at least it is extremely easy to control. It's about at the upper limit of size for proper concealment in a pocket, I think. At 1" thick, it stands out unless you have deeper cargo pockets and a well-made, curving holster that covers the edges well.
They each the smallest of their breed. Each fits in the pocket well. The P3AT and LCP are probably the least well made of the lot, based on the size/durability of the parts used in them. Though, basically the goal of the LCP's introduction was a higher-quality P3AT based on much the same design. (My experience with the P3AT also involves some parts breakages. I have yet to see the PM9 or NAA 380 go belly-up due to various parts failing.)
On the MouseGuns web site, here is a comparison of the basic size/specs of these and other small pistols: click.
Pricing, highest to lowest: PM9 ($600+), P380 (<$600, planned), NAA 380 ($400), LCP ($350), P3AT ($300).
For my money, the LCP and P3AT are worth considering if absolute slimness and low weight are the primary goals to the exclusion of everything else. The P380 will probably be a hit in the same class, though it's not proved itself on the market yet. I had a PM9 and liked it a lot, though my particular one had some growing pains. I'll get another at some point, since the package cannot be matched elsewhere. (Probably the same can be said of the upcoming P380. We'll see.)