My rule of thumb is to match the amp output with the AES power rating of the speaker. That usually gives a good match in my experience and should be relatively safe as long as you're careful. Many of the popular 500W amps produce about 300W at 8 ohms, which makes a 300W speaker a good match. The driver manufacturers test their products to AES standard and publish the results, but some bass cab manufacturers use their own figures, which can be much higher. Look for an AES rating, sometimes stated as rms. Anything else is likely to be 'music power', or twice the AES rating.
Having said that, there's a lot more to it than power ratings. Ideally, you want your amplification to be cruising. So if in doubt, use speakers with a much higher power handling spec than the amp you're using - if you can.