Hey there neighbour (I am Aussie)! I like round pens to be 50 feet in diameter (not sure what that translates to in metres, I just pace it out and mine's unfortunately about 22 feet thus WAY too small) with fences 2m high or higher.
Could you get your hands on some quality arena footing? I like washed river sand (which is basically very fine gravel, so fine it's not ouchy, and it drains REALLY well and has great traction - I rode in a jumping arena that had it as the footing and my horse and I love it), or some people use shredded rubber, or a mix of both. Blue metal mixed with sand can also be a good option but can be dusty depending on the grade of blue metal you use.
Beware that shredded rubber gets VERY hot so if your mini is going to be living in it full time it's not a great option, the heat of it can get so bad that people have burned their feet through their riding boots! If it can do that to a human's feet through shoes, imagine what it would do to a horse's hooves... This is why a lot of people who use it mix it with sand, because that way it doesn't get quite so hot.
My round pen just has a sand footing, which is the cheapest way to do it because you just whack the fence in and then use it until the grass goes. I have used my pen as a diet yard for a pony and had no issues, as long as they have hay they are fine. A slow feeder, feeding straw instead of hay, and soaking it before you feed are all good ways of reducing the sugar intake while keeping up the necessary fibre.
Just as a backup for my reasons for recommending a very large round pen with a high fence rather than a smaller cheaper one with a low fence; PLEASE ensure your round pen is REALLY safe!!
my girl is 15.3-ish at the moment, maybe 16hh by now, and could have died because my round pen isn't tall enough or large enough.