  Massive binaries that merge as compact objects are the progenitors of
gravitational-wave sources. Most of these binaries experience one or more
phases of mass transfer, during which one of the stars loses part or all of its
outer envelope and becomes a stripped-envelope star. The evolution of the size
of these stripped stars is crucial in determining whether they experience
further interactions and their final fate. We present new calculations of
stripped-envelope stars based on binary evolution models computed with MESA. We
use these to investigate their radius evolution as a function of mass and
metallicity. We further discuss their pre-supernova observable characteristics
and potential consequences of their evolution on the properties of supernovae
from stripped stars. At high metallicity we find that practically all of the
hydrogen-rich envelope is removed, in agreement with earlier findings. Only
progenitors with initial masses below 10\Msun expand to large radii (up to
100\Rsun), while more massive progenitors stay compact. At low metallicity, a
substantial amount of hydrogen remains and the progenitors can, in principle,
expand to giant sizes (> 400\Rsun), for all masses we consider. This implies
that they can fill their Roche lobe anew. We show that the prescriptions
commonly used in population synthesis models underestimate the stellar radii by
up to two orders of magnitude. We expect that this has consequences for the
predictions for gravitational-wave sources from double neutron star mergers, in
particular for their metallicity dependence.
