The size of the car speakers you choose should be based on the size of your car and the space where you want to install the speakers. You should also consider the power handling capability of the speakers and how much power your car's audio system can deliver.

If you are replacing the factory speakers, you should choose a size that is compatible with the existing speaker openings in your car. If you are installing speakers in a new location, you will need to choose a size that fits the available space and can be mounted securely.

It's a good idea to measure the space where you want to install the speakers and compare it to the dimensions of the speakers you are considering. This will help you determine if the speakers will fit in the available space.

It's also important to consider the power handling capability of the speakers. Make sure the speakers can handle the power output of your car's audio system. If the speakers are not rated to handle the power output of your system, they may be damaged or may not perform well.

In general, larger speakers are capable of producing lower frequencies with more power, while smaller speakers are better suited to producing higher frequencies with less power. So if you want to improve the bass performance of your system, you may want to consider installing larger speakers. On the other hand, if you want to improve the clarity of the high frequencies, you may want to choose smaller speakers.

I hope this helps! Let me know if you have any other questions.