I have a horse on half acre (we own more than a half acre but I only have use of a half acre). In your position I wouldn't divide it into an less than two half acres, and only let them graze for a few hours no more than 3 or 4 hours a day. If you want grass to live that is, even than it probably is pushing it. On my half acre, my mare grazing 6 hours every day that the weather is good not to wet, or during drought. The grass just stays above four inches tall, but I let it grow to five or six before letting her graze on the same spot (I use temporary fence and a series of gates around her dry lot). She also has hay out when she is grazing, and I let her eat hay before turning her out so she is already some what full.
Horse Keeping on Small Acreage is a great book.