Max 100% attendance is with 19k arena. After that you fill one seat per 2 seats built, until you reach 21k (max is 20k attendance). After that you will fill one seat per 5 seats built and there is no cap (for example 25k arena has max attendance of 20800). Can you build a big arena? Yes! Can you get more attendance if you build a bigger arena? Depending on ticket prices, Yes!
Is it profitable? NO!