All sizes are informative. The actual size of item depends on the manufacturer, country of origin, model, item condition and other factors. Considering that the item is used, its actual size may differ from the one indicated in manufacturer's label.
The measurements provided are taken with the garment lying flat on the floor, fastened. They are the actual measurements of the garment and not the measurements of the person it will fit. You need to double across the body measurements to compare to your own, e.g. if a garment has a 26" armpit to armpit measurement, this means that the coat has a 52" chest, and will be a suitable fit for someone with around a 42"-44" chest - you need to allow a little room for movement, comfort and other layers.