Both of mine get roughly 27 to 33 lbs of a hay a day (a bale of grass hay split between them) during cold months. My mare weighs 1,050 lbs for her best weight and the gelding just shy of 1,000 lbs.
They also get a a little more than half of a small bucket full (one of those 2.5 gal buckets) of chopped alfalfa hay and whole oats, with a vitamin supplement mixed in. It works out to about 1 lbs of oats and 1-1.5 lbs of Alfalfa. The vitamin supplement is designed for horses on no fortified grain or feed, so they get a full spectrum of vitamins and minerals, as well as a probiotic.
They stay a good weight and shiney even in their winter woolies!