Right now, my boyfriend and I are keeping our horses on about 4 acres. I have an older percheron mare out there, and he has a 5 year old quarter pony gelding and a yearling (will be 2 in the spring) quarter horse stud colt. I also have a soon to be 3 year old quarter horse filly (the stud colt's sister) who will be joining them in the summer.
There are 2 fields: the front one is pretty much clear of debris, but there's an old trailer and it's where we park our horse trailers (which we have 3 of). It's about an acre-an acre and a half. The back one is roughly twice the size of the front one, maybe a little more. It's overgrown with mesquite, cactus, and other nasty stuff, but the horses have done a great job of eating the grass down and knocking a lot of the brush down. There's also some old, abandoned (stolen?) cars back there that we're going to get out eventually. We're working on clearing a path big enough to get a truck and trailer back there to haul them off, but they'll be stuck there for at least another couple of weeks.
Right now, we just leave the gate open and the horses all live together and come and go as they please. The gelding is at the top of the pecking order, then my mare, then it's a tie between the stud colt and the pygmy goat. No kidding. With the stud colt getting older (and bigger), we're going to have to seperate him from my mare pretty soon. I'm not too worried about him getting her pregnant now because he's not tall enough to 'reach' her yet, but he will be pretty soon.
I'm thinking that the best thing to do would be to leave the colt and the gelding together because the gelding is keeping him in line, and the two of them are very social. My mare will stay near the other horses, but she's not buddy sour and doesn't mind being away from them. I think she would do best in the back because she's old, lazy, and doesn't get into stuff the way the other two do, so I think it would be safest to have her back there and the other two up front where it's more open and they can run around easier. When my filly comes back, she'll most likely go with my mare.