Usually horses are blanketed to protect them from weather or keep them clean.
Full size blankets can be used to help cool down and dry a sweaty horse, stablize the temperature of an ill horse, or warm one whose hair has been clipped. They can also insulate a horse from severe wind and cold.
Saddle blankets keep the saddle clean and protect the horse from pressure points.
if you have trained a horse and it is sweaty you would put a cooler on it, a thin fleece blanket
I think in the Winter.
winter
If the rider is riding the horse they wear a bridle and a saddle and if they are outside they wear a flymask and / or a fly sheet or nothing and if they're in the stall they will wear nothing or a horse blanket.
it depends on the weather,season, and the kind of blanket
A Large size
Yes, a horse can wear a blanket year round. For cold temperatures, there are blankets of different weights for different temperatures. In the summer, horses can wear an anti-sweat sheet or a fly sheet to keep them clean.
It is bad for horses to wear blankets as you can read in the related link. The only time you would need to blanket a horse is if you rob them of their protection from the cold by clipping them. In this case the blanket is the lesser of 2 evils, even though it is still really bad. To tell if the horse needs a blanket, touch his ears. If they are cold, he is most likely cold. If not, he probably isn't, so leave him unblanketed.
Measure your horse from the middle of his chest to the middle of his tail. The number of inches will be the blanket size.
you can't. the blanket will only go on your horse when the horse is sick.
Winter
If your horse shivers from the cold in the winter, you need a blanket. some horses get cold easy other don't. If you live where you don't get snow I would just let your horses grow there extra hair the natural way, but if you live where it snows you should put the blanket on.