A large part of this answer is going to ultimately depend on the designer of the particular clothes for which you are shopping, but a couple of quick and easy rules of thumb:

1. When the sizes are numbered as they are in the U.S., go up one size. In other words, a person who wears a size 10 in the United States would wear a size 12 in Parisian sizing.
2. Sometimes, shirts and sweaters are sized in terms of T1-T4. When you encounter this, a rough approximation of the breakdown to how those translate to American sizing terms is: T1=Xtra Small, T2=Small, T3=Medium, T4=Large.

As an added precaution, since returning something once you are 5,000 from the point of purchase is somewhat out of the question, try EVERYTHING on before you buy in Paris. In general, despite language barriers, sales associates in Parisian boutiques are friendly and helpful.