Perform a load calculation per the NEC and size the Service accordingly.
Obviously, 800A fuses will be accepted by a 800A switch -- presuming they are the right type.
Your query gives the appearance of someone new to electrical engineering -- as against field wiring of apartments.
As a general rule Service equipment is sized per the Code and fused to the maximum of the frame size. Further, the typical Poco will only run a limited selection of wire sizes for their drops or laterals. You'll want to know what those values are, too.
After the MAIN, typically a fused bolt-switch, each dwelling will have a disconnecting means. These days each dwelling will be normally individually metered. If so, the Poco will have very definite preferences as to locations and meter heights and working clearances.
I do not recommend attempting any design work of this nature unless it is overseen by an experienced engineer or electrical contractor active with your Poco. In this market, a newbie is going to get into trouble.
