The approach for quantitative estimation of asphaltene sizes in crude oils in situ via precise simulation of electron spin resonance (ESR) spectra of the slowly rotating VO2+-containing fragments was developed. The method is based on the correlation between the size of the paramagnetic particles and their characteristic rotational time that can be determined by ESR in situ while incomplete averaging of anisotropic hyperfine interactions is observed. The precise simulation of the ESR spectra of heavy molecules, labeled naturally with vanadyl ions, allows one to find their size distribution in crude oils. In particular, the Method is demonstrated, to be an effective tool for the quantitative determination- of the asphaltene sizes in different oil fractions in situ: