Colloidal particles are irreversibly attached to fluid-fluid interfaces. Such adsorption is depends on the surface charges, wetteablity, and characteristic of trapped partices. Adsorption of solid particles decreases the interfacial tension and consequently stabilizes the interface. Accordingly, solid particles can be good alternatives to replace molecular surfactants. The behaviors of interface-trapped particles and their assembled structures at the interfaces are affected by two competing interactions: electrostatics and capillarities. Typically, the electrostatics interactions effectuate repulsions of the particles due to asymmetric charge distribution across the interface, whereas the capillary interactions lead to attractions to stabilized the system by minimizing the surface free energy. Since the relative strength between these interparticle interactions determines the assembled colloidal microstructures and their rheological properties, it is significant to quantitatively investigate the electrostatic and capillary interactions. In the context of fundamentally understanding the effect of particle size on the interparticle interactions, we fabricate polystyrene particles with controlled size distributions via the microfluidic method, and measure the interactions of between the particles with different dimensions using optical laser tweezers.