To measure your horse, use a tape measure and measure from the center of their chest, to their tail. The number of inches is your blanket size. Ex. Your horse measures 78″ from the center of his/her chest to his/her tail. Your blanket size should be 78. Some blankets still come in 2″ increments, but most come every 3″, so for example the sizes would be 63, 66, 69, 72 etc.
Some blankets have a longer drop to them so that they will wrap around your horse when buckled. Others come with a belly wrap instead of straps.
Leg straps are available on some blankets and not on others. Leg straps can help some blankets to stay put and not slide one way or another as the horse moves about or rolls. Leg straps come with snaps on one or both ends.
If your horse is clipped, or you live in a colder climate, you may want to look at blankets with hoods. Some have removable hoods, and some have the hood attached.
You can also get blankets in various weights. No fill is considered a lite, up to 200grams is medium and 300 and up is heavy. What weight you use is determined by various factors such as the horses age, weight, if they have a thick winter coat, or if they are clipped.
A blanket can be nice to keep your horse toasy warm and comfy, but be careful not to over blanket and make him/her sweat. A good way to judge is check your horses ears, if they are cold, he/she is probably cold.
It is a good idea to check your horse often. Remove the blanket and check for any dirt, debris, skin conditions etc. Wash your blanket, especially if you switch blankets to a different horse. Use a cleaner made specifically for horse blankets as using some detergents will strip the waterproofing out.
You can get sprays to help re-waterproof your blankets if needed.