Collinear laser spectroscopy was performed on the isomer of the aluminium isotope 26mAl. The measured isotope shift to 27Al in the 3s23p 2P○3/2→3s24s 2S1/2 atomic transition enabled the first experimental determination of the nuclear charge radius of 26mAl, resulting in Rc=3.130(15)  fm. This differs by 4.5 standard deviations from the extrapolated value used to calculate the isospin-symmetry breaking corrections in the superallowed β decay of 26mAl. Its corrected Ft value, important for the estimation of Vud in the Cabibbo-Kobayashi-Maskawa matrix, is thus shifted by 1 standard deviation to 3071.4(1.0) s.