The effects of slot width for confined, laminar impinging slot jets of millimeter-scale are considered, including experimental measurements of spatially resolved distributions of local Nusselt numbers measured on a constant heat flux surface. The effects of Reynolds number, nozzle-to-plate distance, and dimensional slot width on the local Nusselt number are investigated for slot nozzle width B values of 0.5 mm, 1.0 mm, and 1.5 mm. Reynolds numbers Re range from 120 to 200, nozzle-to-plate distances H/B vary from 0.75 to 12.5, and the nozzle aspect ratio y/B is 50. Observed are different stagnation point Nusselt number Nuo variations with Re, H/B, and B, where the onset of unsteadiness, and the intermittent flapping motion of the jet column are both associated with important variations to local, stagnation region Nusselt numbers Nuo, as experimental configuration and condition change. The variations of these stagnation-point Nusselt numbers associated with these two modes of unsteadiness are characterized by correlations which provide the dependence upon Reynolds number and normalized nozzle-to-plate distance ratio, H/B, for different dimensional values of B. Also presented are stagnation region Nusselt number variations, for steady, impingement jets at values of H/B less than 4.6–7.8. These are characterized by three separate regimes of behavior, each of which shows significantly different Nuo dependence upon Re, H/B, and B.