the size depends on what kind of money you have. i'm D.III and i just want my stadium to cover the costs of players and staffs, but i'll slowly expand when the money gets in. if you have a good amount of money then either get new players to improve your team or use what you see fit on the stadium. i only went to 6,000 seats coming out of D.IV and now i'll be at 8,100 in about 2 weeks. keep in mind that a change in the staff system is coming so you might want to hold on to excess funds just in case there is a new way to buy staff members.