A solar company will size your solar system by evaluating your annual electricity consumption. Since your energy usage will fluctuate monthly, you will want to find your average monthly usage over the course of a year. Typically, you’ll want to design a system to cover all of this electricity usage. Since the production of your system will also fluctuate monthly, you will want to find that monthly average your system produces as well. By using these averages, the net of your usage and production will cancel out at the end of the year.
Other factors that contribute to the size of your system will be sunlight and weather data for your exact location, shading on your roof (since that can limit the amount of sun hitting the system), your roof pitch (since the optimal angle for solar panels varies by region), and the orientation of your system as it relates to the sun (since that will determine how much sunlight your system can collect throughout a day and over the course of a year).
The size of your house or roof is only a factor if you don’t have any electric usage data, as that data provides a much more accurate way to size your system.