Choose your size based on your body measurements, mainly bust width and hip width. Compare with the finished measurements to get an idea of how Hanna is designed to fit. Hanna has a relaxed fit but will feel different depending on your fabric choice.
The fabric should have at least 30% stretch (especially for the mock turtleneck and cuffs). If your fabric has less than 30% stretch, consider sizing up. You don’t need to size down just because you have a fabric with a lot of stretch (around 60% and more). However if the fabric has lots of stretch and little or no recovery* (which is common if there’s no lycra in the fabric), then consider sizing down.
* No recovery means that if you stretch out the fabric and then release, it doesn’t go back to its original width.