Dear Ahmed,
Choose the collector type on the Solar Water Heating input page. You can either choose a collector from the library (the largest collectors in the library are 12.1 square meters), or you can enter your own parameters for any collector not in the library.
The total size of the system is determined by the collector area and number of collectors. You can design a large system with many small collectors, or with fewer large collectors.
It is up to you to determine what collector to use based on your design.
Best regards,
Paul.