Size: Some books and illustrations show only the moth in a "pinned out" pose, with the fore-wings outstretched and the underwings also outstretched so that the moth looks like a large capital 'X' - in the real world moths do not always rest in this position. It is difficult to equate a picture of an outstretched moth with the one you see which might be 'long and thin' - the modern way, now, is to use the length of the fore-wing as the measurement.
If we use the Length of the Fore-wing as the measurement then it doesn't matter if the moth is resting with wings outstretched or beside the body - or even wrapped round the body. Once moths have emerged from the chrysalis they are full grown and will not increase in size. This means that the length of the fore-wing for an individual species can be given as a range eg., 19-24mm.