Size-dependent stabilities and intracluster reactions of potassium atom and acrylonitrile molecules (AN; CH2=CHCN) clusters were investigated. Previously reported magic numbers (intensity anomalies) of n=3k (k=1-4) using photoionization mass spectrum of K(AN)(n), and size-specific elimination reactions (HCN elimination from clusters of ngreater than or equal to3, and H-2 elimination from n=3 and 6 clusters) were explained by a cyclohexane derivative formation in an intracluster trimeric cyclization (anionic oligomerization) initiated by electron transfer from a K atom in K(AN)(n). To elucidate larger K(AN)(n) structures, unimolecular metastable dissociations of K+(AN)(n) photoions were observed using a reflectron time-of-flight mass spectrometer. A metastable dissociation pathway of n-->n-1 (AN-loss) was predominantly observed for all parent sizes; furthermore, for parent ions with n=6, 9, and 12, pathway of n-->n-3 [(AN)(3)-loss] was also observed. These size-dependent dissociation pathways of photoions are related to structures of neutral clusters since intramolecular bonds are expected to be formed in the oligomerization reactions in neutrals and to be conserved in the photoionization process. Parent clusters that cause the n-->n-1 dissociations have structures in which at least one AN monomer can coordinate without forming any chemical bonds. The observation of n-->n-3 pathways corresponds to the existence of isomers of n= 3k (k=2-4) clusters having k cyclohexane derivatives, which are formed by intracluster multiple trimeric cyclization reactions with 3k AN molecules in neutral clusters. The existence of at least two types of structural isomers (including reacted AN or unreacted AN) in these clusters is shown from these experimental results, and is further supported by calculations of the microcanonical dissociation rate constants for each pathway based on the Rice-Ramsperger-Kassel-Marcus theory.