Argon-gas injection parameters including flow rate and bubble size distribution are important to prevent nozzle clogging, control the mold flow, and reduce bubble-related defects in steel continuous casting. This work employs water-air model experiments and analytical modeling to quantify the behavior and size distributions of gas bubbles in the nozzle and mold during nominally steady-state slab-casting, focusing on gas injection through a stopper-rod with multiple side-channels. Bubble formation, breakup, coalescence, and accumulation are investigated with experiments using a one-third scale water model with the aid of high-speed video recording and analytical models to predict gas pressure, initial bubble size, bubble descending velocity, bubble residence time, and bubble size distribution considering accumulation. In addition, size distributions of the bubbles in the nozzle and mold are quantified by analyzing snapshots of bubble images. Bubbles initiate at the gas channel exit in the stopper-rod after overcoming the pressure threshold, and then expand and elongate until detaching from the stopper-rod surface. After that, turbulent flow breaks up the bubbles inside the gap between the stopper-rod and the nozzle inlet. The bubbles sometimes coalesce with others, and get bigger while flowing down through the nozzle. The bigger bubbles have longer residence time, and accumulate in the nozzle, due to higher buoyancy on them. With higher gas flow-rate, bubble size distribution in the nozzle and mold shows larger average-size and broader size-range. Finally, the validated initial bubble-size model with the water-air model measurements is extrapolated to estimate argon-bubble sizes in molten steel with real caster conditions.