This study computationally investigates the effects of finite size of ion via steric factor and interfacial slip on heat transfer features of pure electroosmotic flow through a rectangular microchannel. Our results show that increasing ion size significantly affects the fluid flow rate with a pronounced reduction observed when interfacial slip is present. The heat transfer analysis reveals new insight into the variation of the Nusselt number under a constant wall heat flux condition. The average Nusselt number ( N u ¯ ) decreases with the steric factor at low Brinkman numbers while it increases at higher Brinkman numbers, indicating a critical Brinkman number that decreases with reduced electrical double layer thickness. Similarly, a critical Peclet number (Pe) is identified, below which N u ¯ decreases with Pe, and above which it increases. Notably, the combined effects of ion size and interfacial slip are particularly important at higher Pe and Debye parameters, leading to a substantial enhancement in heat transfer performance compared to cases with no-slip and point-sized ions. Furthermore, the heat transfer reduction due to the steric effect can be effectively mitigated by adjusting the slip length. This offers a promising strategy for optimizing micro-electro-mechanical thermo-fluidic systems and provides new insight into the impact of ion size and interfacial slip effects in microfluidic and electrokinetic heat transfer applications.