Measurements are taken with the garment lying flat. Shoulder - 19 measured straight across the front, from shoulder seam to seam.
Chest - 20.5 measured straight across the front, from underarm seam to seam. From the top of the collar to the bottom hem (down the back).
The best thing to do is to compare the measurements here to the measurements of a similar item that you own and that fits you well. We are always happy to help. Please ask if you have any questions.