The halter must fit the properly. If it is too small, your horse will find it extremely uncomfortable. It will then try to take it off by rubbing against fence posts or similar. If, on the other hand, the halter is too big, there is a risk that your horse will get caught on feeders, fence posts, etc.
Halters are available in at least three sizes: pony, thoroughbred and warmblood. Depending on the manufacturer, there may be other sizes as well. Most pasture and stable halters are adjustable in length and width with two buckles. The noseband should reach down to your horse's nostrils. It must rest at least a finger's width below the cheekbone to prevent chafing.
Adjust the width of the halter so that there is room for two or three fingers between the horse's nose and the noseband. For grazing, it is better to tighten the halter a little. There should also be at least two fingers between the throat strap and the lower jaw.