Nanofluids (NFs), suspension of nanoparticles (NPs) in conventional fluids such as water and ethylene glycol (EG) have been found capable of providing enhanced heat transfer compared to pure heat transfer fluids. Among several factors influencing thermo-physical properties of NFs including thermal conductivity (TC) and viscosity, particle size plays an essential role. Due to limited literature data on the impact of particle size on thermo-physical properties of NFs as well as inconsistent results in the literature, there is a serious need to perform a detailed study on the influence of particle size on thermo-physical properties of NFs. For this purpose a study was carried out using copper nanoparticles (Cu NPs) and copper microparticles (Cu MPs) to investigate, experimentally and theoretically, the effect of Cu NPs and MPs on thermo-physical properties of Cu NFs and Cu microfluids (Cu MFs). A series of stable Cu NFs and MFs with various NP/MP concentration (from1 wt% to 3 wt%) were fabricated by dispersing Cu NPs and Cu MPs in EG. The use of additives/surfactants was avoided to study the real impact of Cu NP/MP. The physico-chemical properties of Cu NFs and MFs were analyzed by various techniques and instruments including Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM) and Dynamic Light Scattering (DLS) and Infrared Spectroscopy (FT-IR). The thermo-physical properties (TC and viscosity) of NFs and MFs were measured at different temperatures between 20 and 40 ^oC. All Cu NFs and MFs showed higher TC and viscosity compared to the EG base liquid. The viscosity test results showed Newtonian behavior for NFs and MFs. Our study exhibited that NFs with Cu NPs revealed higher TC than those containing Cu MPs at the same particle concentration and temperature. For TC, Maxwell predictive equation and for viscosity of NFs/MFs, Kriger-Dougherty correlation was applied to compare the experimental results with the estimated values. Our findings on the physic-chemical, thermal and thermo-physical properties of the NFs/MFs, containing Cu NPs/MPs are presented in detail.