The barn I used to ride at had a 50' pen and it worked fine for us, being that our lungeline is 25', and we don't use the whole line...
We saw one on a farm by us that was made from tposts with caps, and then strapped to the posts were the driftwood type rails that you often see in farm/cattle fencing. I have a whole corral of those i have to take down, so i'm going to use that method. Eventually i'll put up a wood one when i get around to putting up the fencing in the upper field where the pen will be, next to my barn in the nook of a field.
Are we asking about a roundpen? 60' is the minimum suggested for riding work. A lungeline is really too small for good long term work. It's hard on their joints. I've gotten plenty of young horses worked on the lunge too much that broke down early from it. My healthiest horses have always had the least amount of lunging and circle work while growing up. For that reason after i got all my current horses broke in my little round pen made of panels I built an 80' diameter one from tape fencing. http://www.horseguardfence.com/shop_intus/cat53.php
I also have a 200x180' arena with 2 rails of electric tape and 1 plastic rail. Get some tposts caps and it's cheap, effective, and safe. Or it could be done with wood posts.
It's being made from tposts with caps yes, and instead of electric tape I'm using all the old driftwood/corral wood that i have to take down anyway from the two corrals that are in disrepair. It's more temporary because we're going to be re-doing it next year when we fence the upper fields. It's going to be 50' this one because it's just going to be used for the pony, and the sheep will get fed in there.