there is, of course, a bureau of standards for clothing sizes-- but roughly-- Juniors-- odd numbers sizes-1 to 15
for younger women-- narrower in the waist and bust, and slightly shorter (Neck to waist, waist to hem) they tend to be for younger women. Misses even numbered sizes 2 to 18 or so. slightly larger in the waist than juniors-- actually the standard from which other sizes are based on. (difference between juriors and misses- is like the difference between regular and "trim fit" in mens clothing) Petites-- can be any size-- just cut shorter-- neck to waist, waist to hem, and sleeve. (since women's clothes don't always come in Long, Reg and Short--Tall women, and very short have the worst of it..)
Worst case is a thin women about 5 foot tall, with large breasts-- she might need a juniors size 3 or 5 skirt--but a Misses 8 for a jacket-- to fit her bust--only now the shoulders, and sleeves would be way to big, and even the back of the jacket wouldn't fit right!

"half sizes" 12½ to 22½-- fuller-- especially in waist and bust.(mens "full cut") Womens 16 (or 18) to 26.. large sizes.. Think of Roseanne-- or Cathrine Manheim.

Size 12 is the standard-- everything is cut larger or smaller based on its relationship to size 12. -- it used to be size 10-- but as america keeps getting larger.. they moved the standard up a notch..