We report on the measurement of freeze-out radii for pairs of identical-charge pions measured in Pb-Pb collisions at √ s NN = 2.76 TeV as a function of collision centrality and the average transverse momentum of the pair k T . Three-dimensional sizes of the system (femtoscopic radii), as well as direction-averaged onedimensional radii are extracted. The radii decrease with k T , following a power-law behavior. This is qualitatively consistent with expectations from a collectively expanding system, produced in hydrodynamic calculations. The radii also scale linearly with dN ch /dη 1/3 . This behavior is compared to world data on femtoscopic radii in heavy-ion collisions. While the dependence is qualitatively similar to results at smaller √ s NN , a decrease in the ratio R out /R side is seen, which is in qualitative agreement with a specific prediction from hydrodynamic models: a change from inside-out to outside-in freeze-out configuration. The results provide further evidence for the production of a collective, strongly coupled system in heavy-ion collisions at the CERN Large Hadron Collider.