Car hi-fi speakers

Hello dear community, I have a question and need your advice. I have bought a new radio (Blaupunkt Barcelona) and am now looking for suitable coax speakers. There will be no power amplifier. The diameter goes up to 20 cm, installation depth 80mm. Firstly, I was wondering whether the larger the diameter, the better the sound? Or secondly, whether the lower the RMS wattage, the better the sound, so that the car radio comes closer to the output power. So clearly: 1. does it sound better when the diameter of the speakers is larger; when RMS and Watt number are the same? 2. do the speakers sound better if the output power of the car radio, e.g. 25 watts RMS, is the same as the speakers 25 watts RMS? What indicators should I look for when buying if I only want to drive the coaxial speakers with the car radio and I can choose diameters of 16-20 cm? I am looking forward to your answers!