Start With Consumption
The starting point for sizing is usually your average monthly electricity consumption or bill, which gives a rough sense of how much generation would be useful.
Your installer will typically ask for a few recent electricity bills to understand your usage pattern.
Then Factor in Constraints
Available shadow-free roof area, roof orientation, local sunlight conditions and your budget all influence the final recommended capacity.
For grid-connected systems, sanctioned load and net-metering limits set by your DISCOM may also cap the maximum permissible size.