What should a horse field be like?

Im just worried my horses living conditions arent right. If they arent then i will move her right away because i dont want to risk injury or anything like that. But the field is about the size of a football field.. maybe smaller but the further you go out into the field the less poopy and mushy it gets but when it rains there are big puddles and mud and squish everywhere. theres gr to graze on but not much. so what are ideal conditions anyway?