Honda uses a proprietary speaker mounting system, closest to a 6.5" speaker in our cars. If you are trying to avoid work and just have something that drops into the factory location, the easiest thing to do will be to get yourself some factory Honda speakers. They fit, are designed to work with the factory radio, and they plug right into the factory wiring.
Aside from that, you can fit most aftermarket 5 1/4" speakers with an adapter (readily available from companies like scosche and others), or use most 6.5" speakers with a custom built spacer ring. (mounting depth is a concern here, as the trunk springs run directly under the rear deck).
Either way, both options will be more work than just tossing in a stock set of Honda speakers.
The speakers themselves are 4 ohm units, and the head unit puts out a little under 15W RMS. Meaning that you should look for something with high efficiency and a low power rating (generally the aftermarket companies cheapest speaker lines).
If you ask me, if all you are looking for is getting sound in the rear... get yourself a set of stock speakers, toss them in, and be done with it. No need to make things into more of a project than necessary.
Hans
