Welcome Hattrick manager. If you login on HT-Tools, you will have full access on the options of the website.
Size according to your Fans Club

Fans Mood
Size according to the total number you want

Seats Current Arena Seats to add Arena Size
Basic Seating
Seats Under Roof
Seats in VIP
Total capacity
Weekly cost
Cost of Construction
Time of Construction (in days)
Arena Calculator
Info: This tool will help you to identify the ideal or desired size of your arena. You may use the fan members and the fan mood or just your desired arena size with the default or your own percentages. The time of construction can vary by up to one day in base of the daily updates.