Totally agree that the bigger the better applies to most arenas. But depending on what you have to do to construct one, it can get pricey quite quickly. When we bought our farm earlier this year, it came with a small-ish “arena” - it had decent drainage and had an OK surface (sand over M10) but it was too small for my needs and it was kind of a weird egg shape which I wanted to fix. I expanded it to 100 x 150, which required a fair amount of site work, new drainage work, and I put in an all weather footing so I could use it even after a big rain. Turned out great but even with that fairly modest size, it was over fifty grand.
