Cuprous oxide (Cu2O) nanocubes were fruitfully synthesized via facile solvothermal route. The crystalline nature and structural confirmation of the synthesized nanocubes was revealed by XRD and Raman studies. The oxygen defects and luminescent abilities were explored by PL studies. The FTIR metal–oxygen vibrations present in Cu2O nanocubes was observed at 510 cm−1. The typical nanocube morphology and the decrease in particle size with NaOH concentration increase was revealed by SEM images. The particle sizes of the best performed nanocubes (2 µm) have been tested for antimicrobial properties. The particle size effect on improved diamagnetic behavior of 5 × 10−3 emu/g was reported for 6 µm scale range. The enhanced antibacterial properties against Gram-positive (Bacillus thuringiensis) and Gram-negative (Pseudomonas aeruginosa) bacteria were reported.