We are building a small arena, primarily for dressage. It is 80’ x 150’. I plan to have two boards, so am trying to decide what height to have the top rail? I will eventually (hopefully) add the low dressage fence inside the larger one.
It’s nice to have it big enough to keep horses in in case you need to turn them out there. You may not ever plan to do that but it happens. So I’d say one at 2’6" and one at 3’6"-ish maybe?
Also if you ride alone its nice if they are trapped inside a fence you fall off and can’t run willy nilly all over the countryside.
It’s also nice not to have other horses wandering through when you are intently working on something. At 150 long i’m guessing that you don’t plan on going much above First. In any case I’d leave enough space between your actual arena, and your dressage arena so that you never rely on the outside fence rather than your outside leg.
I frequently tease students that without the menage walls, they would be so far away that I could never see them. 
for a smaller ring, tall fences can make it feel smaller and more closed in. I have a standard dressage ring (80x200 ish) and don’t have any fences bordering it. We have railroad ties around the longsides as a visual aid and to keep the footing contained. The “real” fence is about 10-20 feet away in every direction, so if I come off, horsey isn’t loose (if I choose to close the gate which is left open 99% of the time). I couldn’t free jump in there, or turnout, but I like the open feeling of the space, and it definitely keeps me or my horse from relying on the rail.
Thanks everyone. I would have loved to have a longer arena, but our property is very hilly, so I was lucky to find an area 150’ long to level without tons of dirt work - it took quite a bit even for this one. I’ve been riding in it without any fence for the past few months, but need a fence as it is in my pasture, and it’s annoying to have a horse wander through when I’m riding!
My arena is 3 board fencing, top rail is 5’6".
My arena is 3-board fencing and 4’6". I suppose it could be 2-board (one at 2’6" ish and one at 4’6"), but I’m not sure I’d want anything lower if you ever want to turn a horse out in it. But if all you want is a visual barrier outside of your dressage ring I suppose it wouldn’t matter if it was tall.
