Our G. Inglese shirts are sized in neck sizes, in centimetres. The sizes are not chest sizes in inches. The two numbers are unrelated.
So you would need to refer to your usual shirt size (neck size). A typical US neck size 15.5" would be roughly a size 39 (15.5 x 2.54). The collar will be slightly larger when fresh out of the box - it is normal and is because the shirtmakers are taking into account potential shrinkage.
The fit of our G. Inglese shirt is moderately slim through the body. Most people take their usual size but some of our customers prefer to take the next size up for a more comfortable fit.