Where I ride, our outdoor arena is about 90 x 200 and our indoor is about 80 x 200. At a minimum, you'll want the area of a small dressage arena, which has enough space for flatting, lungeing and one or two small jumps, which would be about 60 x 100. Bigger is better in this case though, you can do more with a larger ring.
The type of footing you choose should be good for your horse's legs, rsomewhat dust-free and long lasting. Ultimately, you want the best you can afford. Sand is a popular choice but it can create a lot of dust, erodes faster and is the deepest of all footings. You may want to create a combination of sand and soil, etc. Sand can also get really muddy after rainstorms and such.Stone dust settles and drains well, doesn't erode and is not as dusty as sand. It also provides nice firm but forgiving footing for the horse. Rubber footing has also been gaining in popularity. It is resilient, dust-free, freeze resistant, no watering, and low maintenance. At my riding camp we have a rubber ring which is almost everyone's favorite to ride in. Before you decide though, visit some other facilities and look at their footing, and then talk to suppliers of footing.
I assume that when you say fences you mean the fence surrounding the perimeter of the ring, so that being said I recommend that you build at least a 4 foot fence if not higher. If your fence is just 6 inches off the ground horses are sometimes more liable to try and jump the fence. (I know this from experience).
Good luck with your arena!