I have a Hejnar D20 on my EOS-M right now and I think it fits pretty well. It's 1.5in wide (needed to clamp) and 1.25in long (front to back). It has a ridge on the back (about 0.1 - 0.125in) to prevent rotation so it sticks out about that far from the back of the camera.
It does not quite extend all the way to the front of the body, but it's close. It stops before the lens mount begins.
One thing to note, although I don't think it matters too much, is that the bottom of the body is not flat all the way from front to back. You can see gaps between the plate and body. This will be the case with any plate - except those that have a pad on the top.
If you look on the bottom of the body around the tripod mount you will see some raised bumps in front of and behind the hole. These fit inside the slot in the plate. If your plate only has a round hole and no slot, you may not be able to seat the plate as closely to the body as you'd like.
Mounted centered around the tripod hole, the plate still allows you to open the battery/SD compartment without removing it. This will probably be the case with most plates, but you may want to check before buying one.