  The production of brighter coherent XUV radiation by intense laser pulses
through the process of high-harmonic generation (HHG) is a central challenge in
contemporary nonlinear optics. We study the generation and spatial propagation
of high harmonics analytically and via ab initio simulations. We focus on the
length scales defining the growth of the harmonic signal with propagation
distance and show that the well-known coherence length limits HHG only for
relatively low driving intensities. For higher intensities, the photoionisation
of the medium, naturally accompanying HHG, leads to essentially transient phase
matching and laser frequency blue shift. By systematically taking both of these
factors into account, we demonstrate that the behaviour of the harmonic signal
at higher intensities is defined by another length scale -- the blue-shift
length. In this generation regime the XUV intensity at a given frequency first
grows quadratically and then saturates passing the blue-shift length, but the
total harmonic efficiency continues growing linearly due to the linear increase
of the harmonic line bandwidth. The changeover to this generation regime takes
place for all harmonic orders roughly simultaneously. The rate of the
efficiency growth is maximal if the static dispersion is compensated by
photoelectrons near the centre of the laser pulse. Our theory offers a robust
way to choose the generation conditions that optimise the growth of the
harmonic signal with propagation.
