There's no international standard for clothing sizes. The system of dress sizing used in the United States of even numbers or of XS-XL is completely different from the Asian systems of sizing. If you go shopping in Tokyo or buy clothing online from an Asian source, this can lead to a huge headache or leave you stuck with a dress far too big or small. It's important to know the size conversions so you find a perfect fit.
In Japan, watch out for lettered sizes (XS-XL), as these tend to run small. If a dress made in Japan is labeled as a medium, for example, it's probably a small in the United States. Japanese manufacturers also use two separate numbered systems. The first is an arbitrary number system like our 2-4-6-8 system in the United States, and the second relies on the actual measurements of those sizes. 5-7-9-11-13-15 correspond to an American XXS-XS-S-M-L-XL, respectively. 36-38-40-42 correspond to an American XS-S-M-L, respectively.
In Korea, you'll find a simple numbered system. 44-55-66-77-88 correspond to an American XS-S-M-L-XL, respectively.
In China, watch out for two separate and more tricky systems. These two systems differ because they rely on the measurements of different parts of the clothing. An American small is 160-165 or 84-86. A medium is 165-170 or 88-90. A large is 167-172 or 92-96. Extra large is uncommon and therefore lacking in real standard measurements, but is generally 168-173 or 98-102, depending on the manufacturer.