Laminar heat transfer in self-cooled liquid metal blankets can be enhanced by increasing the aspect ratio of the ducts. To determine the potential benefits of elongated rectangular ducts, numerical simulations of MHD fully-developed flow and developing heat transfer were performed. Results show that as the aspect ratio increases (i.e., the ratio of the side wall to Hartmann wall length), the peak velocity and side layer flow quantity increase, which leads to enhancement of the average heat transfer coefficient along the side layer. The pressure gradient decreases with increasing elongation, providing an added benefit. However, results of the heat transfer analysis also indicate that the non-uniformity along the heated wall and the peak wall temperature both increase as the aspect ratio increases, due to smaller velocities in the corners and near the interface between the side layer and the core. The net benefit to reactor blanket design is therefore uncertain, because designs are usually constrained by the peak structure temperature. At fixed velocity, elongated ducts always have higher peak temperatures. However, the reduction in pressure gradient allows the designer to increase the average velocity, which improves thermal performance due to lower bulk temperature rise as well as higher wall heat transfer coefficient. Calculations show that peak temperatures can be reduced relative to the square duct case with lower pressure gradient by optimizing the velocity. Elongated ducts may suffer from larger pressure stresses due to geometric factors. Thermal stresses are also likely to increase, owing to the increased thermal gradients in the walls. Overall, it is difficult to guarantee that elongation will provide improved performance without a more detailed design analysis.