At about one-quarter of the size of a standard Stylophone, the mini is a bit cumbersome as a key chain novelty fob, but it is probably going to make your keys difficult to lose. It runs on three AAA batteries and was available in at least two color schemes: black/chrome and white/chrome.
I have also seen photos of a model in ivory/gold, but I have never actually seen it for sale.
The 1.5 octave keyboard is quite playable and the sound is surprising loud for such a small speaker. The mini has only one timbre and no vibrato switch, fine-tuning knob, volume control or earphone jack.