The smallest indoor horse arena that you can ride comfortably in that I've seen is 60'x120'.
As far as price, don't be surprised at how much it costs...materials and labor are both expensive these days. Also, a big factor is your weather, e.g. How much wind and/or snow load is there. A building that size has a lot of roof to support.
Having built pole barns in the last couple years ourselves, I don't see how you could build anything
for less than $5/sq ft doing all the labor yourself, so 60'x120' puts it in the $36,000+ range. You don't want to cut (too many) corners on the construction or you'll be paying for it on maintenance/repair costs in the long run.