The best way to find the right size for you is to select a shirt of yours, to measure it and to choose in the chart which one has the closest measurements.
Lay your shirt flat on the ground and smooth the fabric out so that there are no wrinkles. Pull the fabric and measurement tape firmly, but don't stretch.
For the chest, waist and hips areas : Measure straight across from the left of the shirt to the right of the shirt.
To determine the lenght: Measure from the highest point down to the bottom of the shirt.