High-performance heat-retention agents for multifunctional green agricultural films are today largely suitable to increase the production yield as well as to save energy. Here, an adapted ammonia releasing hydrothermal method was used to produce a series of micrometer-sized carbonate-layered double hydroxide (CO3-LDH) precursors of sizes ranging from 1.32 μm to 8.64 μm by simply adjusting the feeding Mg2+ concentration from 0.80 mol L−1 to 0.20 mol L−1. From these pristine LDH materials, μm-sized dihydrogenphosphate-intercalated LDHs (H2PO4-LDHs) were prepared by an anion-exchange method. The structure, the platelet size, and the associated selective IR absorption properties of the H2PO4-LDH and the derivative H2PO4-LDH/EVA composite as well as the related visible transmittance and the photostability of the H2PO4-LDH/EVA film were investigated. The results show that the selective IR absorption in the wavelength range of 7–14 μm enabling the heat retention of the H2PO4-LDHs and H2PO4-LDH/EVA composites depends on the corresponding number-averaged particle size of H2PO4-LDH in the range of 2.01 μm to 8.80 μm. Compared with EVA, the H2PO4-LDH/EVA composites demonstrate a significant improvement of selective IR absorption, while maintaining acceptable visible transmittance, and similar photostability. An optimized particle size of H2PO4-LDH of ca. 5.85 μm leads to 60% selective IR absorption and 64% selective IR absorption when dispersed in EVA, while the polymer free of filler exhibits less than 50% absorption in the 7–14 μm IR domain.