Are there any optimal proportions to the arena, which brings maximum attendance?
Unfortunately not (yet).
Also, are there any construction fees other than those related to the addition of each seat?
Nope, the contruction fees are included in the seats buying price.
A last one is: does anyone knows which the drop in attendance, by raising the price of tickets?
Higher prices leads to less attendance. You can change you ticket price once per month to experience the effects of a pricechange. If your arena got a lot of capacity you might try a lower price, but if your arena is sold out most of the time, raise the prices.