The nuclear charge radius of ^{32}Si was determined using collinear laser spectroscopy. The experimental result was confronted with abinitio nuclear lattice effective field theory, valence-space in-medium similarity renormalization group, and mean field calculations, highlighting important achievements and challenges of modern many-body methods. The charge radius of ^{32}Si completes the radii of the mirror pair ^{32}Ar-^{32}Si, whose difference was correlated to the slope L of the symmetry energy in the nuclear equation of state. Our result suggests L60MeV, which agrees with complementary observables.
