Heterogeneous superconductivity onset is a common phenomenon in high-Tc superconductors, both of cuprate and iron-based families. It is revealed by a rather broad transition from metallic to zero-resistance state. Usually, in these strongly anisotropic materials superconductivity (SC) first emerges in the form of isolated domains. It results to anisotropic excess conductivity above Tc, and the transport measurements give valuable information about the SC domain structure deep inside the sample. In bulk samples this anisotropic SC onset gives an averaged approximate shape of SC grains, but in thin samples it also suggests the averaged SC grain size. In this paper both interlayer and intralayer resistivity is measured as a function of temperature in FeSe samples of various thickness. To measure the interlayer resistivity microbridges of FeSe are produced using FIB. A considerable raise of superconducting transition temperature Tc is observed as the sample thickness decreases: Tc raises from 8 K in bulk material to 12 K in microbridges of thickness 40 nm. We apply analytical and numerical calculations to analyze these and earlier data and find the SC domain aspect ratio and size in FeSe consistent with our resistivity and diamagnetic response measurements. We propose a simple and rather precise method of estimating the SC domain aspect ratio from Tc anisotropy in samples of various small thickness. The relationship between nematic and superconducting domains in FeSe is discussed. We also generalized the analytical formulas for conductivity in heterogeneous anisotropic superconductors for the case of elongated SC domains of two perpendicular orientations with equal volume fractions, corresponding to nematic domain structure in various Fe-based superconductors.