The best way to purchase is to go by the actual measurement of the item. When I measure a blouse, I lay it on a flat surface and measure it straight across the bust line from underarm to underarm. Then, I take this measurement and multiply it by two to get the entire circumference of the blouse. This is the bust measurement. The bust measurement is not the same thing as your bra size, since your bra size is not the fullest part of your chest.
To determine your measurement for blouses, I recommend measuring a top that you already own that fits you the way you like. To measure, lay the top on a flat surface and measure it straight across the bust line from underarm to underarm. Multiply this number by two, and you have your measurement. Using it as a guide is the best way to ensure a good fit. If your measurement is 44 inches and the item you are interested in has a small bust measurement of 43 inches and a medium of 45 inches, then you probably need to size up to a medium unless you prefer a closer fit.
Since different styles are sometimes sized differently, you may need a size small in one style, but may need a medium in another. You may also need to consider how closely that particular style is supposed to fit to achieve the look you would like.
If I don’t have measurements listed on a particular item listed, feel free to send me an email to ask. I am happy to measure for you. I want the item to look lovely on you when you receive it!