We are both in Ireland aren't we trigger, so I can probably give you a good indication. I just had an arena about the size of a 40 metre circle built... Its small, because that's all I have, in terms of space. I got it done properly, with crumb rubber as the base, and its bloody FANTASTIC! Its perfect for a pony like trigger, because there are no long sides for the pony to tank off on... Its just a big circle. Mine cost roughly... 6500 euros to build. It was awesomely cheap for something that good!
I love it for doing dressage in, and you can put a small jump or two in there, but only if the pony is small. My horse is 16.2 and still growing, so he is simply too big to jump in there :P