What factor directly influences both engine size and power?

The number of cylinders in an engine is a crucial factor that directly influences both engine size and power. Each cylinder in an engine is responsible for the combustion process, which generates power. When there are more cylinders, there is typically a greater total displacement, which allows for more air and fuel to mix and combust. This larger volume increases the engine's power output because it can produce more energy per engine cycle.

Larger engines with more cylinders can also run at lower RPMs while producing the same power as smaller engines with fewer cylinders, contributing to better performance and efficiency in various driving conditions. Therefore, an increase in the number of cylinders generally correlates with an increase in both size and power, making this choice the correct answer.

Factors such as engine age, engine brand, and fuel type may influence performance or longevity but do not have a direct impact on the actual size and the inherent power potential of the engine as determined by its design and number of cylinders.