I (Heather) tried them on! :)
These distinctions have more to do with aesthetics than the measurements (see below).
Ultra-Short = never needs to be combed, the shortest wigs on the market, top usually cannot be tucked behind the ears, extremely short nape (< 3”)
Short = may occasionally need to be combed, top can be tucked behind the ears, short nape (no measurement cutoff here)
Mid-Length = Falls anywhere on most people from right below the jaw to up to 2” below the shoulders
Long = Wig falls below the shoulders (so, longer than mid-length, though some in this category could be considered longer mid-length) to around the mid-back on most women
Ultra-Long = FRONT of the wig (which is usually shorter due to layering) is very long and extends past the breasts of most women, BACK of wig extends beyond the waist on most women