In this paper we have investigated the effect of nanometric grain size modulation on the behavior of magnetoimpedance (MI), magnetoresistance, and magnetic properties of a series of single-phase nanocrystalline colossal magnetoresistance La0.7Sr0.3MnO3 (LSMO) manganites all synthesized through chemical route “pyrophoric reaction process.” MI measurements were carried out at room temperature (T=300 K) in the ac signal frequency (f) range of 40 kHz-110 MHz and in dc magnetic field (H) range of -2.5 kOe≪H≪+2.5 kOe, respectively. The experimental results show that MI% increases with the increase in nanometric grain size (Φ) for this series of samples. This behavior can be explained in the light of classical electrodynamics, which relates this effect with the dependency of skin depth on external magnetic field and applied ac signal frequency. Interestingly, MI of about 92% was obtained for the 28 nm LSMO sample at a frequency (f) of 10 MHz and at H=2.5 kOe. The same sample also showed a maximum MI sensitivity of about 1%/Oe near 60 Oe dc magnetic field at f=10 MHz. Magnetization measurements were carried out in the applied magnetic field range of -5 kOe≪H≪+5 kOe and the experimental results reveal that spontaneous magnetization decreases with decrease in particle size. These nanometric LSMO sam- ples also showed a paramagnetic-ferromagnetic transition near TC=360 K. For comparison, we have also presented room temperature low-field magnetoresistance (LFMR) study of this series of LSMO nanoparticles. This study reveals that within this particle size range, 14–28 nm, LFMR decreases with decrease in particle size, which in fact mimics the variation of MI with particle size.