The nature of convective organization remains elusive, despite its importance for understanding the role of clouds in climate systems. This study reports a new type of large-scale structure formed by the self-organization of deep moist atmospheric convection in radiative-convective equilibrium. To understand the natural behavior of convection unaffected by the computational domain, we conducted cloud-resolving simulations by systematically increasing the horizontal domain size to approximately 25,000 km. We found that if the domain side length exceeded 5,000 km, the domain-averaged thermodynamic fields and the horizontal characteristic length converged in quasi-equilibrium; the cloud aggregation area exhibited a mesh-like pattern, analogous to the shallow convective organizations despite their different scales. Its characteristic length is estimated to be approximately 3,000–4,000 km. The results suggest that this length scale is related to the upper-limit size of mesoscale convective systems or the scale of supercloud clusters in a real tropical atmosphere.