Nanoparticles’ properties are determined by their size and structure. When exposed to external pressure P, their structural properties change. The improvement or degradation of samples’ properties depending on time is particularly interesting. The knowledge of the influence of structural-size effect, aging time, and pressure on compounds’ behavior is essential for fundamental and applied purposes. Therefore, the first attempts have been made to shed light on how the properties of the Mn-containing perovskites change depending on them. The La_0.8-xCd_xNa_0.2MnO₃ (x = 0 and 0.05) nanoparticles of various sizes, 20–70 nm, prepared at t_ann = 500–900°C and the La_0.7A_0.2Mn_1.1O₃ (A = Na⁺, Ag⁺, K⁺) nanoparticles analyzed at the t_ann = 900°C studied before and after aging time and additionally under pressure. After 3 years, their structural properties underwent significant changes, especially for the smallest nanoparticles. The phase transition temperatures increase with nanoparticle size, time, and pressure. The aging time has the strongest influence on the Curie temperature T_C for the smallest and most magnetically inhomogeneous nanoparticles with dT_C/dP ≈ 100 K/GPa. Conversely, the structural-size effect and external pressure have the greatest influence on the biggest and most magnetically uniform nanoparticles with dT_C/dP ≈ 91 and 16 K/GPa, respectively. After 3 years, the biggest nanoparticles demonstrate the most stable phase transition temperatures with improved magnetocaloric parameters near room temperature. These structural-size effect, aging time, and pressure are powerful instruments to tune the nanoparticles’ phase transition temperatures and magnetocaloric effect. These outcomes may have implications for the whole class of perovskites and could initiate a new mainstream.