I remember reading somewhere, also, that you need to set fuses differently if you are using an external crystal vs an external resonator, so you may want to check into that as well...
Starting on page 26 of the datasheet, "System Clock and Clock Options":
...details what to set fuse bits for various kinds of clocks, internal, external, crystal, or resonator. I seemed like the settings used for an external crystal or resonator were the same, whether you were using full swing or low power oscillation.
You would probably want to have this document handy if you were building a standalone implementation; I have a feeling that if you know the specs of your crystal, it would probably be fairly easy to figure out what size caps to use exactly. If you didn't have the specs to your crystal (and just "guessed" at the capacitor value - 22pF is a common choice, but you could try 18pF, 20pF, and 33pF), though, you might find that operation at low power might not work, versus "full swing" operation...?
I'm not really certain, to be honest; I just remember reading somewhere that fuse setting were different if you were using a crystal vs using a resonator, but maybe my memory is faulty, as it doesn't appear that the datasheet backs me up there...