Petite was the same as it is today. Women who are short wear petites so they don't have knee-length dresses hit at mid-calf, or extra fabric bunching above the waistline. It makes sense to me.
Misses was everything from size 0-16 or 18, but often you can't find anything smaller than a 6 or larger than a 16. I'm not sure where truly tiny people are supposed to shop.
Women's clothing was about 16-18 on up to the biggest, depending on the clothing line. Sometimes marked with a W, a 16W would be bigger around than a 16.
Now, instead of Women's clothing, many manufacturers, stores, and websites are calling it Plus size. I guess that's more polite than calling it "chubby" or "husky" like children's clothing used to be labeled. I get most confused when I am browsing online. I can go into a store and quickly determine where the larger-size clothing is, but online the terms get all confused. I'll see an ad for an adorable "Women's Coat" and click on the site, only to find out it is made no larger than a size 14.
Is this more of an international standard? It makes more sense, really, because the whole "Misses" thing has no logic to it. I am just having trouble at times sorting it all out. Some websites clearly say "Plus Size" which I like--as long as there is an adorable coat. I found a red one on Groupon, and have been sorely tempted to order it.
For clothing, I am more prone to shop locally, so I can try things on, so this shouldn't matter. However, I like to get ideas of style and put things on Pinterest from the comfort of my computer chair.