round pen building
I Built my own round pen, literally with 2 or 3 days help from a man, it was struggle to do it but I got it done , and it's great. Except when winter came and the woodchip surface became lethally slippery ( bought very cheaply locally, with too much bark- avoid like the plague!Get real equine woodchip if you are going for it at all). As for straw bales- good idea! I'd advise AGAINST electric fence as some horses will barge right through it and can get tangelled up- have seen this happen. Not a pretty sight. Also, the solid look to a straw bale wall is much better psychologically, no so much temptation to reach over to grass greenr on oher side. With unpowered electric fencing horses can easily duck their heads under the tape for a much. If you wanted to make it more long-term you could make it out of stra bale and then plaster the walls- this is a form of building that is popular in eco-circles, cheap and easy. Do a search on the internet for 'straw bale buildings'.
the standard diameter for a round pen is 50 foot, or 60 for draught very big horses. Good luck!