Rule number one is to make sure the people you’re buying from allow returns!Sizes can be confusing, especially when manufacturers use S, M, L, and XL rather than dimensions. It's not a good idea to base your choice on clothes you already own because one maker's M can be another maker's L. Every bike jacket manufacturer should have a size chart. Some put it right there on the product page, while others, for reasons we can't understand, make it more challenging to find. When measuring yourself, think about the kinds of clothes you'll be wearing under your jacket. There's a big difference between a summer T-shirt and a bulky wool sweater. Finally, get someone to help you measure, and stand normally!