Cells survive in extreme environmental conditions by accumulating small organic molecules called osmolytes. While we have reached a consensus on the role of osmolytes toward macromolecular conformational stability at a single-macromolecule level, there is a lack of clarity on how osmolytes might influence macromolecular aggregation, an important feature to maintain cellular homeostasis. In this regard, here, we explore how a popular osmolyte trimethyl amine N-oxide (TMAO) individually dictates the self-assembling propensity of hydrophobic and charged macromolecules. Our computer simulation-based results reveal that the TMAO-induced self-aggregation of hydrophobic macromolecules, relative to that in neat water, is strongly dependent on the macromolecular length scale. Specifically, a free energy-based analysis indicates that the self-aggregation propensity of hydrophobic macromolecules in aqueous TMAO relative to neat water follows a nonmonotonic trend. When compared with neat water, TMAO promotes hydrophobic self-assembly at a shorter length scale while discourages hydrophobic self-assembly at a larger length scale. The overall nonmonotonic trend is found to be entropy driven. A molecular-level analysis suggests that length-scale-dependent preferential exclusion/binding of osmolytes (relative to water) from/to macromolecules in the process of aggregation holds the key to the nonmonotonic behavior. The overall result is found to be robust, even in the presence of the charge distributions on the macromolecules.