Results on the correlation between microwire dimensions and anisotropy distribution in the surface region and subsequently on their influence on the magnitude of the GMI response are reported. Calculated results show a strongly nonlinear dependence of surface region anisotropy on the metallic core diameter. Theoretical results allow one to understand the complex relation between GMI response and microwire dimensions. A suitable criterion for the optimization of the magnitude of the GMI effect has been found. Glass thickness has little effect on the magnitude of the GMI effect in optimized microwires