You don't have to guess the impedance of a speaker, all you need is a multimeter set to measure low ohmic resistance. The impedance of those size speakers could be in a wide range from 8-60 ohms or thereabouts, I have many dozens of such speakers, and they do look like headphone drivers, close to 32ohms as transistor495 said, not sure why but headphone drivers tend to use that clear plastic diaphragm material. There is no point is guessing power output at that size it's irrelevant, if you can hear it it's okay.
If you want to evaluate a speakers output power then the easiest method is to simply test it with a good ear in the enclosure it will be in, increase the volume until you can hear distortion that you don't like, back off 10% and calculate practical power from that.
Cell phones have low power small "speakers" like that. They produce squeaky sounds with no low frequencies.
audioguru, I am publicly upset with your statement as such is not the case, and your name being 'audioguru' you should know better than making such broad misleading statements especially to someone trying to learn something, provide information or keep your negativity to yourself.
Aside from the frequency limitations of the size of the speakers making lows very difficult to produce cell phones and the devices these types of speakers are put into are seldom constructed around a good audio cavity. That speaker WILL produce solid low frequencies in the same power output range as it does it's mid and high tones IF it's put in the proper speaker cabinet, which would be many times larger than the speaker for free air speakers.
Them being headphone drivers they did produce bass, because the air cavity between the ear muff and the ear itself acted as the resonant cavity.
It sure isn't high fidelity but I have a multimedia cell phone that was designed with the speakers in mind, enough so that placed flat on a solid object the internal air cavity and the resonance of solid platform make something not completely horrible. Lost power once and placed it at the mouth of an empty kleenex box just for giggles, amazing what an air cavity can do.