The Width or the “Last” of a ski boot refers to how many millimeters across the boot measures across the balls of the feet (at the base of the toes) in the reference size 26.5 for men and 25.5 for women. As the boot gets longer in size it gets slightly wider to accommodate the larger foot.
Junior width boots are specific to a junior foot.
Narrow boots can range anywhere from 95-99mm (in the reference size) so they can be sculpted and shaped by a boot fitter to match the shape of the skiers foot. These are typically high-performance or race boots.
Medium width boots typically measure 100mm-103mm (in the reference size). These boots are usually performance or advanced boots.
Wide ski boots accommodate a foot that measures 104-106mm (in the reference size), they can range in performance from expert to beginner. Wide boots are common for beginners and intermediates since the extra width is generally more comfortable. Luckily for those with wide feet, more options available in models for expert skiers now than ever before.