You have two options. one is use LJ units and another one is real units. For LJ units take spherical particles with dia - 1 units for 4 nm, 3 units for 12 nm and 5 units for 20 nm. or else you take real units: like 4 nm for 4 diameter. Moreover, both cases water molecules diameter is constant. Then simulate using LAMMPS software.

Hi Goutam Kumar Biswas

You can describe a particle of roughly a spherical size by using the LJ potential (pair_style lj/cut) on LAMMPS.

Dealing with nanoparticles with the LJ interaction might not always be the best option. You can instead use the COLLOID package on LAMMPS and use pair_style colloid (https://docs.lammps.org/pair_colloid.html) -- You can find more on the implementation of the potential for nanoparticle systems in this paper (Article High Energy Density Flexible and Ecofriendly Lithium-Ion Smart Battery