  We introduce a suite of thirty cosmological magneto-hydrodynamical zoom
simulations of the formation of galaxies in isolated Milky Way mass dark
haloes. These were carried out with the moving mesh code \textlcsc{AREPO},
together with a comprehensive model for galaxy formation physics, including AGN
feedback and magnetic fields, which produces realistic galaxy populations in
large cosmological simulations. We demonstrate that our simulations reproduce a
wide range of present-day observables, in particular, two component disc
dominated galaxies with appropriate stellar masses, sizes, rotation curves,
star formation rates and metallicities. We investigate the driving mechanisms
that set present-day disc sizes/scale lengths, and find that they are related
to the angular momentum of halo material. We show that the largest discs are
produced by quiescent mergers that inspiral into the galaxy and deposit high
angular momentum material into the pre-existing disc, simultaneously increasing
the spin of dark matter and gas in the halo. More violent mergers and strong
AGN feedback play roles in limiting disc size by destroying pre-existing discs
and by suppressing gas accretion onto the outer disc, respectively. The most
important factor that leads to compact discs, however, is simply a low angular
momentum for the halo. In these cases, AGN feedback plays an important role in
limiting central star formation and the formation of a massive bulge.
