Fluorescence enhancement and particle-size control of organic fluorophores are of general interest owing to their wide applications. We recently developed a convenient five-component reaction (5CR) for the synthesis of a novel class of racemates, C-6 unsubstituted tetrahydropyrimidines with strong aggregation-induced emission (AIE) properties. The copper-induced fluorescence enhancement and particle-size decrease of the racemate dimethyl 2-(4-hydroxy-3-methoxyphenyl)-1,3-diphenyl-1,2,3,6-tetrahydropyrimidine-4,5-dicarboxylate (THP-1), one of the 5CR products, were investigated in this paper. THP-1 shows useful AIE properties, which have attracted great attention recently, and the unusual size-independent emission (SIE) properties reported in our previous work. The fluorescence intensity of THP-1 increased upon adding Cu2+ and a good linear relationship between the fluorescence intensity at the peak position and the Cu2+ concentration was observed in the range of 0–80 μM of Cu2+. The fluorescence response to Cu2+ is highly selective over other common transition metal ions. Most importantly, the suspension particle sizes of THP-1 were found to gradually decrease upon adding Cu2+. This method for controlling organic particle-size is completely different from conventional ones. The investigation results of the influences of Cu2+ on the optical properties and particle sizes of THP-1 in ethanol–water suspensions as well as the molecular stacking mode in the single-crystal of THP-1 indicate that the copper-induced fluorescence enhancement and particle-size decrease are expected to result from the coordination-induced dissociation of intermolecular O–H⋯O bonds connecting non-emissive groups. In addition, THP-1 shows no cytotoxicity in EC109 cells at concentrations below 30 μM. These useful and interesting properties of THP-1 are expected to be very helpful for new probe design, theoretical research on the relationship between intermolecular hydrogen bonds and optical properties, as well as practical applications in organic particle-size control and chemical/biological fluorescence probes.