In 2011, the ALICE Collaboration measured the size of the system created in Pb-Pb collisions at a centre-of-mass energy of 2.76 TeV per nucleon pair. ALICE confirmed that the QCD matter created in Pb-Pb collisions behaves like a fluid, with strong collective motions that are well described by hydrodynamic equations. The fireball formed in nuclear collisions at the LHC is hotter, lives longer and expands to a larger size than the medium that was formed in heavy-ion collisions at RHIC. Multiplicity measurements by the ALICE experiment show that the system initially has much higher energy density and is at least 30% hotter than at RHIC, resulting in about double the particle multiplicity for each colliding nucleon pair (Aamodt et al. 2010a). Further analyses, in particular including the full dependence of these observables on centrality, will provide more insights into the properties of the system – such as initial velocities, the equation of state and the fluid viscosity – and strongly constrain the theoretical modelling of heavy-ion collisions.