We analyse the sizes and specific angular momentum content of galaxies in the colibre cosmological hydrodynamical simulations spanning two orders of magnitude in mass resolution. We compare the predicted size–mass and angular momentum–mass relations to a broad range of observational measurements spanning redshifts z = 0 to 4. At z = 0⁠, colibre reproduces observed size–mass relations over the sampled mass range ⁠, and for multiple size definitions, including two- and three-dimensional stellar half-mass radii, half-light radii across several wavelengths, as well as alternative measures such as baryonic half-mass radii and characteristic radii defined by stellar surface density thresholds. The simulations also recover the observed segregation of galaxies in the size–mass plane by morphological type and star formation rate, and reproduce the distinct, approximately parallel sequences followed by star-forming discs and quenched spheroids in the stellar specific angular momentum–mass plane. The angular momentum content of star-forming colibre galaxies matches that of observed systems out to ⁠z ≈ 1.5. At higher redshifts, massive galaxies (⁠⁠) in the simulations are somewhat smaller than observed, and the separation between star forming and passive populations in the size–mass plane is reduced relative to observations, while at lower masses the agreement remains good. This apparent discrepancy may reflect the effects of dust attenuation, which is neglected in our analysis and may preferentially obscure the central regions of observed systems. Overall, our findings highlight the close connection between galaxy size, angular momentum, and morphology over cosmic time.

Stellar haloes form through the disruption of satellite galaxies over time, making them a promising observable for constraining galaxy merger histories. We use a dynamical decomposition technique to isolate the stellar haloes of Milky Way-mass galaxies in the Eagle simulation and study their relationship to the merger histories of their hosts. We define the stellar halo as the stellar mass that is bound to the central subhalo but not associated with the disc or bulge components of a galaxy, and we quantify their merger histories using the most significant merger since. Surprisingly, we find that the fraction of a galaxy's total stellar mass in the stellar halo, is not a reliable indicator of its merger activity. Contrary to common assumptions, disc galaxies with low do not necessarily have quiescent merger histories. In fact, roughly one quarter experienced a merger at with a satellite whose stellar mass was at least 10 per cent of the host galaxy's stellar mass. These galaxies undergo mergers with satellites on circular orbits that are roughly co-planar with the pre-existing disc and thereby avoid contributing mass to the stellar halo. Instead, such mergers build thick extended discs and supply fresh gas that often triggers a significant episode of star formation in the disc. Our results suggest that disc galaxies with low-mass stellar haloes, such as the Milky Way, can have varied and active merger histories, and that stellar haloes may not be a reliable tool for inferring galaxy merger histories.