Good Fit: A blanket that fits well will not be too tight or loose in any one area. Most blankets have many adjustable elements, but the length must be correct. The blanket should cover your horse from withers to tail without too much overhang, and without pulling in any section.
Too Large: A blanket that is too big for your horse can move around on his body, causing a potentially unsafe situation if he steps on it or gets his leg caught in a strap. A loose blanket is also less effective at keeping out winter weather.
Too Small: If your horse has a blanket that is too small, it may leave some parts of his body exposed to the elements. Tight-fitting sections can also rub out his hair or lead to sores. A small blanket will also restrict your horse’s movement, making it more difficult for him to run or walk through deep snow.
Neck Opening: If the neck opening is too small, it will not only be uncomfortable for your horse, but may restrict his movement. Additionally, the pressure can rub your horse’s coat or even create sores if he wears a too-tight blanket regularly. If you are finding you have to adjust the chest buckles so wide that your horse is not completely covered, then your blanket does not fit properly. Notice that the blanket is pulled tightly around the horse’s shoulder (above middle.) However, if the neck opening is too loose, the blanket will move around on the horse’s body, and the extra space allows cold air, rain and snow to get underneath the blanket (above right.)
The neck opening should fit snugly so that the horse’s chest isn’t exposed, but loose enough that he can move his neck up and down comfortably. If your horse is the type that likes to nibble on his blankets, a closed-front blanket may be a better option so that he isn’t able to remove it. Correct fit is even more imperative with a closed-front blanket as there is generally little to no adjustability.
Surcingle Straps: Adjust the belly straps so that you can easily slide your hand between the straps and your horse’s belly. Too tight and they will chafe. Too loose and your horse could get a leg caught in them.
Leg Straps: If your blanket has leg straps, cross them around each other as shown to keep them from rubbing on your horse’s legs. Leg straps should not be too tight, but again, make sure they are not so loose that they could get caught on something. Leg straps are not always necessary, but they will help keep your horse’s blanket from falling off to the side.
This article courtesy of HorseChannel.com