Sometimes it just has to be small. For this use case HiFiBerry has build the DAC+ Zero, that fits perfectly onto a Raspberry Pi Zero. It does not have the advanced features as volume control, EEPROM, ultra-low-noise regulators, but still offers a very good sound quality and 192kHz/24bit playback.
This is their smallest board in the Raspberry Pi Zero form factor. It can be used not only with the Raspberry Pi Zero, but also with the bigger Raspberry Pi B+/2 and 3.