First, I found an article to help get you started: How to Build a Horse Fence: A Step by Step Guide
You will Want 8 inch diameter, 8 foot tall posts. I have them on my property and I have 4 acres fenced in. (I bought pipe fencing.)
You will want 2 ft. x 12 ft. horizontal boards, although I really like the look of the long posts used AS boards, too.
REMEMBER: BOARDS go on the INSIDE of your FENCE!!!
The BO bought fencing from a horse place that went under, used them to fence in the paddock that I rented, and where he put them on the outside of the posts, the horses leaned on them and they came loose.
You can break your arms using a slam in the ground and pull apart post hole digger. I much prefer an auger--sorry can't find a picture of mine, which is not electric--BUT you can rent one that runs on gas, and save yourself a LOT of labor.
You put the 8 ft. post 3 ft. into the ground. The gravel sounds great, but I've pulled up 60yo posts from one of the previous farmers who fenced the property in for cattle and the below ground wood wasn't rotten. I think they try to sell you this to get you to buy more stuff. The ONLY rotten posts were those set in cement. (DON'T get me started on the broken off metal fencepost supports set 3 ft in cement that I've had to remove!!!!)
Always buy treated wood, but honestly, it's about the safest material you can find.
ONE MORE THING: Something I never would have thought to do but a previous owner DID--
Fence in an area in front of your barn. Horses make a beeline for an open gate. I cannot tell you how many times in the last 15 years I've had a horse escape to that area (we call it "The Inner Sanctum") when I wasn't home, and I didn't loose him to a busy highway.
In addition, We have separate turnouts and a small (55 ft x 65 ft.) area that used to be a garden, but is now a training area. We have a rectangular 3 acre North pasture, the training area, the area in back of the barn, the Inner Sanctum, and a 3/4 acre South Pasture. It REALLY helps to not just have one big fenced in spot. My 3 horses do not NEED all of that space, so you can start much smaller, and then add on.
Hope this helps!