We investigate the heat transfer characteristics of electromagnetohydrodynamic electroosmotic flow in a rectangular microchannel by incorporating the steric effect along with the interfacial slip. The steric effect is represented by the bulk volume fraction of ions and is accounted through the steric factor ( υ ) . Our analysis systematically examines variations in the average Nusselt number ( N u ¯ ) by varying key parameters, including the Hartmann number ( H a ) , Debye parameter ( κ ) , lateral electric field parameter ( S ) , Joule heating parameter ( G ) , and dimensionless slip length ( β ) . The results reveal that the conventional point charge assumption leads to significant deviations in heat transfer predictions, overestimating N u ¯ for thicker electric double layers and underestimating it for thinner ones, with a critical Debye parameter ( κ =62.31) marking this transition. Notably, the deviation becomes more pronounced with increasing slip length and Hartmann number. Moreover, N u ¯ increases monotonically with H a for lower S and follows a non-monotonic decreasing-increasing trend for higher S . The presence of Joule heating not only enhances heat transfer at higher H a but also amplifies overprediction in N u ¯ due to the point-size assumption of ions. These findings provide crucial insights for optimizing micro-electro-magnetic-mechanical systems where precise thermal management and flow control are required.