Molecular size effects on the site specific photofragmentation of a core excited molecule were investigated by exciting the N and O K-shells for a series of CH3CO(CH2)(n)CN (n = 0-3) in which the O and N atoms were separated by successively inserting a CH2 group between the two atoms. The photofragment ions coincident with total photoelectrons were observed by means of a reflectron type time-of-flight mass spectrometer. Little difference was observed in the fragmentation patterns between the N and O K-edges in exciting CH3COCN, CH3COCH2CN, and CH3CO(CH2)(2)CN. Clear discrimination was observed in the long chained CH3CO(CH2)(3)CN.