Isotope shifts in dielectronic recombination spectra were studied for Li-like Nd57+A ions with A=142 and A=150. From the displacement of resonance positions energy shifts δE142150(2s-2p1/2)=40.2(3)(6)meV [(stat)(sys)] and δE142150(2s-2p3/2)=42.3(12)(20)meV of 2s-2pj transitions were deduced. An evaluation of these values within a full QED treatment yields a change in the mean-square charge radius of δ142150/r2/=-1.36(1)(3)fm2. The approach is conceptually new and combines the advantage of a simple atomic structure with high sensitivity to nuclear size.