Manufacturers of amps usually like to quote really big numbers. If they quote instantaneous peak power, divide by four for a realistic RMS value. If an amp is working off 12 volts into 8 ohms, it is unlikely that the speaker will ever see more than 10watts, and if the amplifier LS connections are a LS line and the other side to ground, then about 5 watts is even more likely. As a handy way of muddying the issue, some speakers are intended for use in HiFi situations with more than one drive unit per box - and they get quoted as "siutable for use in system up to n watts".
As said before, get one about the size you want, mount it so that the noise coming off the back does not cancel the noise coming off the front, and if you can stand the weight, get the one with the knobbliest magnet.