  We present Director's Discretionary Time multi-frequency observations
obtained with the Jansky Very Large Array (VLA) and the Very Long Baseline
Array (VLBA) of the blazar PSO J030947.49+271757.31 (hereafter PSO J0309+27) at
$z = 6.10\pm0.03$. The milliarcsecond angular resolution of our VLBA
observations at 1.5, 5 and 8.4 GHz unveils a bright one-sided jet extended for
$\sim500$ parsecs in projection. This high-z radio-loud AGN is resolved into
multiple compact sub-components, embedded in a more diffuse and faint radio
emission, which enshrouds them in a continuous jet structure. We derive limits
on some physical parameters directly from the observable quantities, such as
viewing angle, Lorentz and Doppler factors. If PSO J0309+27 is a genuine
blazar, as suggested by its X-ray properties, then we find that its bulk
Lorentz factor must be relatively low (less than 5). Such value would be in
favour of a scenario currently proposed to reconcile the paucity of high-z
blazars with respect to current predictions. Nevertheless, we cannot exclude
that PSO J0309+27 is seen under a larger viewing angle, which would imply that
the X-ray emission must be enhanced, for example, by inverse Compton with the
Cosmic Microwave Background. More stringent constraints on the bulk Lorentz
factor in PSO J0309+27 and the other high-z blazars are necessary to test
whether their properties are intrinsically different with respect to the low-z
blazar population.
