Tree size is a fundamental attribute of forest stand, reflecting the role and status of individuals in the process of forest ecosystem. We established three permanently fixed plots (A, B and C) in a mixed forest of pineoak in the Nanpan River basin, southwestern China in 2016-2018. We analyzed the spatial structures of snags of different tree size using pair correlation function g(r), mark correlation function k_mm(r), mark variogram function γ(r), and distance distribution function k^th neighbor D^k(r), with the purpose to disclose their relationships at stand level. Our results showed that: (1) distribution pattern tended to be clumped at small scale (r = 0-5 m) when the minimum measured diameter (mmd) of snags was small. Random distribution pattern, however, dominated the majority of scales (r = 0-20 m) with the increasing of minimum measured diameter (mmd = 5-8 cm). (2) Tree size approximated aggregation at small scale (r = 0-1 m) when the minimum measured diameter was equal to 1-2 cm. Otherwise, it maintained a random pattern. (3) Tree species showed a clump pattern when the minimum measured diameter was smaller than 4 cm. On the contrary, it was in or became random at each scale. (4) The abundance of neighbors decreased and the distances between any pair increased within r = 0-10 m with the increasing of the minimum measured diameter from 1 cm to 8 cm. These results suggest that the minimum measured diameter of snags is closely related to distribution pattern, tree mark and distance, implying the smallsized snags of conspecific have a clump pattern while the largesized ones are likely to be distributed randomly. A uniform minimum measured diameter of snags should be used when analyzing spatial structure.