Basically the size depends on ticket price so its tough to say X is the right size.
That said, the only way to make money consistently long term is through arena expansion and winning games so if you make baby steps every week it will pay off long term.
Check out what other teams are charging in your division and what kind of attendance they are getting and try to figure out an optimal price from that to almost fill your arena. (if you are sold out you are probably charging too little)