This study examines the influence of public and managerial ownership on carbon emission disclosure (CED) in private companies in Indonesia, with firm size as a moderating variable. Guided by legitimacy theory, the research employs a quantitative approach using secondary data from 616 private companies listed on the Indonesia Stock Exchange (IDX) during the 2021–2023 period, resulting in 1,826 firm-year observations. Using panel data regression and the Fixed Effect Model (FEM), the results show that public ownership has a statistically significant positive effect on CED (coefficient = 0.0247, p = 0.031), suggesting that increased public shareholding enhances environmental transparency. Managerial ownership also positively influences CED (coefficient = 0.0094, p = 0.029), although the impact is less pronounced. Firm size significantly moderates both relationships. The interaction between public ownership and firm size (coefficient = 0.0059, p = 0.047), and between managerial ownership and firm size (coefficient = 0.0546, p = 0.045), demonstrates that larger firms amplify the effects of ownership structure on disclosure. These findings highlight the importance of both ownership configuration and firm scale in driving sustainability reporting practices in developing countries. The study offers valuable insights for policymakers and stakeholders aiming to promote environmental accountability and strengthen corporate governance.