Some clothing items can fit a wider range of sizes making it easier to get an accurate fit for the person you are gifting to.
Robes are a great clothing item because they really can fit okay even if they are a bit loose. A robe or afghan will generally fit over a few size ranges so are a great clothing item to purchase if you are not quite sure of a person's exact size.
If in doubt pick them up a gift card instead. Fashion is a touchy area anyway but choosing a wrong size can make your choice a disaster. Play it safe where clothing items are concerned.