The HGL down coats are form-fitting unless otherwise marked as loose fit in the description. The following guide are based on fit snugly as recommended. If you want to put chunky sweater or such, you might want to have extra room and go a size up.
If you want to double layer the HGL pack-able coats, leaving about extra half size room should be enough.
We also posted some actual size in some of the coats below.
Measurement of some of the coats. You can measure your existing coats as a reference. Just remember, HGL coats are usually form-fitting unless the description state loose fit explicitly. What that means is that the sleeve and shoulder are nicely fitted, one of the feature a high end coats have and gives the sleek look.