The dynamics of a single bubble rising in a viscous Newtonian fluid was investigated both experimentally by a particle image velocimetry (PIV) device and numerically using the free-energy-based lattice Boltzmann (LB) model. The rise velocity, bubble shape and flow field were considered for various bubble volumes in axisymmetric flow conditions. Experimentally, the flow measurements by the PIV device revealed the wake increasing with the bubble volume. Such an evolution is linked to the deformation of bubble shape from spherical for small bubbles to flattened at the bottom for large bubbles. The LB simulations compare satisfactorily with our experimental data for both the bubble shape and drag coefficient over the range of Reynolds number ($ 0.033 \leq Re \leq 1.8 $). With a more extended flow structure around the bubble compared to experiments, the two-dimensional approach shows some limitations in its quantitative description. Fully three-dimensional simulations are necessary, especially for bigger bubbles with $ Re > 1.8 $.