I drove minis this week in a 70X170 indoor arena and it was really nice to have that long straight drive on the rail and being able to really let them move out. I did note though, that after we messed around and got warmed up that when I started working more on collection and head set and bending and circling and doing figure eights that I was only using about 1/3 of that arena, about a 60X60 area maximum.
We use someone elses arena for our indoor work and though it is close by, it is a pain to load up horses and carts to go over there. I'd love my own arena but we have issues with city restrictions on size and the type of structures allowed. Even though we have 10 acres I'm restricted to adding what would amount to a covered round pen to my barn. I am trying to decide if something that small is worth spending the money on. For us the only reason it would work is because we have other nearby options for when we need more space. I'd end up using it just for doing in-hand training, ground work, conditioning and jumping. I'd still be driving outdoors or at the bigger arena when I wanted more space and wanted to be able to just get out and drive.
I see you asked about the 42' wide? It is tight, but would work for minis. With that radius you can bet they will learn to collect and bend and give turning. The length definitely would work.
My dream arena would be 100X200 or like the one I was in this week 70X170 which are typical sizes for arenas at boarding facilities where there frequently are several big horses being worked or ridden at the same time.
My more realistic personal arena to build suitable for horses big or small would be 60X90 - which is a very, very common size for a privately owned arena.
My personally more realistic ideal indoor area based on our city restrictions and what we are allowed to build, would be a covered round pen (google that and you will see some very cool portable structures designed specifically for round pens) if it had good footing I'd be very happy to have that space to work with my horses and not have to worry about snow, ice, rain, wind. etc. I don't care if it isn't heated either, I'm finding it difficult to work unclipped minis in an indoor arena that is heated they get way too hot. If I had my own heated arena I'd probably at least do a trace clip and keep them blanketed during the winter to make it easier to keep them cool and not sweating too much inside.
