Silica nanoparticles have shown extensive applications as drug carriers in cancer treatment, and studies have revealed that the bioactivities of drug-loaded silica nanoparticles in special tumors are associated with the size of these nanoparticles. However, methods for the simultaneous synthesis of multi-sized silica nanoparticles have rarely been investigated. Herein, we proposed a Christmas-tree microfluidic platform that incorporated sharp-corner microstructures into a four-layer serpentine microchannel for the simultaneous synthesis of multi-sized silica nanoparticles. The first layer included two inlets serving as the inputs of the microfluidic platform and three serpentine branches serving as the inputs of the second layer. The number of branches increased from three to six when the number of layers increased from one to four. Numerical simulation results showed that the proposed microfluidic platform could significantly increase the mixing efficiency while ensuring a linear distribution of outlet concentrations. According to the experimental demonstration, ammonia concentration played a dominant role in the linear increase of sizes of synthesized silica nanoparticles. The ammonia concentration at the six outlets followed a linear distribution ranging from 1.96 to 3.91%. The synthesized silica nanoparticles from the six outlets were measured with six different sizes, satisfying a linear distribution ranging from 112.3 ± 35 to 232.7 ± 46.3 nm, with the polydispersity index below 0.1 for all of the six outlets. The influencing factors were also investigated and the results demonstrated that the silica nanoparticle size decreased as the reaction temperature increased or as the reaction time decreased. In a demonstration of drug screening, drug-loaded nanoparticles with six different sizes exhibited the ability to significantly reduce the viability of cancer cells as well as the negative correlation between the cell viability and the nanoparticle sizes. This finding holds significant implications for precision cancer treatment, thereby improving clinical efficacy and reducing the damage to normal cells.