For example, your measurements are as follows:
Bust: 38 inches
Hip: 44 inches
Recommendation:
Abaya Bust: 38 + 6 = 44 inches.
Abaya Hip : 42 + 8 = 50 inches
Using the values 38 inches (bust) and 42 inches (hip) the above size chart, the corresponding size is Small.
If the resulting value is in between two sizes (let's say, between S and M) choose the bigger size. It is important the abaya is not too tight and you can easily make a rakah in it.

The USA dress sizes are to provide a reference point. Do not order based upon the dress size but on the true body measurements. Dress sizes are unreliable for basing your abaya purchase on them solely.