I have a very small woodchip which I can jump a single fence in it is slightly bigger then a 20metre circle(sorry don't know the exact sizing) it was originally built for me free jumping my youngsters.
However I have a big 20 jump course set out in the field which is to big as I forget sizing.
I would go with a size you feel happy enough with say like warmup arena size BUT build it so when you get more finances it can be enlarged further. This is what a friend of mine is doing granted hers is outdoor
To give a horse your heart guarantees a love that will last forever undamageable