It may look big-ish now, but when you get in there you'll really feel how small it is. I think this barn is being sold as more of a "barn" meaning for stalls and horse care, rather than riding.
It's do-able, anything really is for what you can afford, but I wouldn't recommend getting anything smaller than 75x100 especially if you're going to do some serious work, including jumping! The horses just don't have enough room to "stretch out".
That's not to say it isn't do-able! It's good that you don't have more than two people using this arena because you probably cannot fit any more riders! I'd recommend looking into something bigger if you do vigorous flatwork, training or especially jumping (I'd also suggest not putting more than one or two jumps in here as well).
If you think that your horses will be okay in this tight environment, then by all means go for it but make sure to exercise in a more open space once in a while, maybe in a field, outdoor ring, trail, or an empty paddock so your horse can stretch! In my opinion, I would get the biggest ring I could afford! Hope I could be of help!:)