This paper studies the effect of particle size on heat transfer in nanofluids flowing through a circular tube using computational fluid dynamics (CFD) simulations. The simulations examine heat transfer in the developed region of the tube for different particle sizes in the nanofluids. They find that decreasing particle size increases heat transfer effectiveness due to increased surface area and Brownian motion effects. The paper addresses common queries about obtaining fluid properties for different nanofluid concentrations and defining particle size in the simulations. It also notes that generating plots from simulation results can sometimes be problematic.