I threw her into the pasture in August, and she was still pawing through the snow to get to the last of the grass in December.
I'm in the process of learning how to maximize grass growth for this year right now, but personally, I wouldn't think you'd have an issue keeping one horse on 3/4 of an acre. You'll have to divide the pasture and rotate, and keep up with manure, but it's do-able.
My friend down the road has 6 horses (one being a draft), and she has her horses on 24/7 turnout. They're always in a good grass turnout. And she has five acres. So yeah, with management, a small acreage can work just fine.