You have come to the right place because a ton of vintage dealers here have TONS of shoes in size 5.5/6 and sometimes have a hard time finding people who will buy them based on size! I know Lizzie (fuzzylizzie) usually has tiny ones because she has tiny feet as well so naturally has radar to find them.
I have the same problem. (not with shoes, I was "blessed" ha ha and am quite ample in the foot size dept. LOL) I am 5' and I have a very small frame/narrow shoulders, tiny waist, short arms and legs, but am full figured everywhere else. So I am really not a "traditional" petite size as far as modern clothing goes, and the one pair of jeans I have found that fits me pretty well is actually not a petite size but a "short". And the jean makers keep making the waists bigger and bigger so I was shocked i found a pair! The size 0-4 ranges are starting to have 30" waists!
I have found that if I look for 70s blazers especially in petite sizes, they fit me to a T. Reason being is that the they are a narrower cut acrossed the shoulders and the arm holes are a bit higher In comparison to the bust line , so I don't look like I have all this extra material under the arms. And since the jackets are a bit more fitted, I look taller lol. I usually wear a size 10 sometimes 12 in 70s jackets.
As far as proportions 40s/50s work out pretty well for me. I have found a sizing trick...Look for the half sizes (i.e., 15, 17, etc whatever works for you and so forth as they are were made more for petites/juniors). I have found the shoulder to waist and the overall proportions much better on the whole.
I unfortunately because of my shoulders, etc, have a hard time wearing the early 60s Jackie kennedy era stuff. Everything was so much more boxey and cut too generously in the shoulders...at least on everything that I have ever tried on.
Chris