To fully understand the capacity of clustered network-MIMO systems and analyze the system performance (throughput, energy-efficiency or quality of service), one must have an analytical expression of the system capacity or capacity bounds. In this paper, the impact of cluster size on the downlink network-MIMO system capacity is analyzed considering inter-cluster interference (ICLI) based on the 1-dimensional Wyner model. For the nonfading channels, the lower and upper bounds of the per-cell capacity with ICLI are derived. The per-cell capacity with ICLI demonstrates a linear growth versus the cluster size in the interference-limited regime due to ICLI. The lower and upper bounds are generalized to a 2-dimensional cellular system. The ICLI turns out to have a great impact on the system capacity when the cluster size is small. Introducing Rayleigh fading channels, assuming the number of users in each cell is sufficiently large, a lower bound of the per-cell ergodic capacity with ICLI is derived.