  We present half-light sizes measured from H${\alpha}$ emission tracing
star-formation in 281 star-forming galaxies from the KMOS3D survey at 0.7 < z <
2.7. Sizes are derived by fitting 2D exponential disk models, with bootstrap
errors averaging 20%. H${\alpha}$ sizes are a median (mean) of 1.19 (1.26)
times larger than those of the stellar continuum, which due to radial dust
gradients places an upper limit on the growth in stellar size via star
formation, with just 43% intrinsic scatter. At fixed continuum size the
H${\alpha}$ size shows no residual trend with stellar mass, star formation
rate, redshift or morphology. The only significant residual trend is with the
excess obscuration of H${\alpha}$ by dust, at fixed continuum obscuration. The
scatter in continuum size at fixed stellar mass is likely driven by the scatter
in halo spin parameters. The stability of the ratio of H${\alpha}$ size to
continuum size demonstrates a high degree of stability in halo spin and in the
transfer of angular momentum to the disk over a wide range of physical
conditions and cosmic time. This may require local regulation by feedback
processes. The implication of our results, as we demonstrate using a toy model,
is that our upper limit on star-formation driven growth is sufficient only to
evolve star-forming galaxies approximately along the observed size-mass
relation, consistent with the size growth of galaxies at constant cumulative
co-moving number density. To explain the observed evolution of the size-mass
relation of star-forming disk galaxies other processes, such as the
preferential quenching of compact galaxies or galaxy mergers, may be required.
