Most sellers will give the exact bust, waist and hip measurements in inches. Another factor to take into consideration is the length from shoulder seam to waist. Taller and shorter women will be aware of this necessity. Have someone do this measurement for you. Measuring on your back, run the tape from top of shoulder to waist and compare to the item, do the same from waist to knee. In 1960 and earlier a petite woman was more common than today. If the skirt is too short, ask the seller if there is extra hem and if it will mark if lowered. Another measurement to consider is shoulder span. If your seller does not have these listed, email and ask.
Factor in the fabric type and assess if there is some give or stretch there. Many of these older fabrics such as cotton, linen and wool, do not have any leeway. Even if they do, do you really want the look to be stretched to the max? Items sized by Small, Medium, Large or Plus tell you nothing. I’ve seen many size large shirts that wouldn’t fit a 3rd grader and I’ve seen those that would fit a football player. There is no consistency in that type sizing. You will find charts on the internet that give you SML guidelines but that is mainly a guessing game. Stick to actual measurements in inches.
Sizing for vintage shoes is pretty simple. Most sellers measure in inches along the inside of the inner sole. Width is measured on the outside, ball of the foot. Once again you have to take into consideration the style of the shoes. For extreme pointed toes some length has to be knocked off. Heel width is also important for many of us, running narrow to wide. Shoe sizes have not changed that much but as you know from shopping in the mall a size 7 isn’t always a size 7. Measure a pair of your perfectly fitting shoes with that tape measure, and compare to what is stated in the listing. If no size in inches is given, ask before you purchase.
For outerwear, common sense reins. Once you have the exact measurement in inches, leave a little room for clothing that is to be worn underneath. Will you be wearing heavy sweaters or a simple silk dress...makes a big difference.
Hats are basically sized in inches only, style being the major factor. If you are purchasing a pillbox hat, head size does not really matter since it perches on top of the head. Most are worn inside and not for extreme weather so you don’t really even need to consider security or placement factors. Hats that fit down over your head are a completely different matter. Measurements are taken on the inside around the sweatband. Your head needs to be measured exactly where you want the sweatband to fit. For a cloche style hat, measure even lower as shown in this example.
So next time you're shopping for that perfect vintage apparel item make it work by making sure it will fit!