The solar panels are used in many different applications, from small application like street solar light up to large application like satellite.
We can confirm that solar panel size will be indicated based on the solar system application available space installation area, as if you have a small area in which you will install the solar system, then small size solar panel with high efficiency will be considered which will cost a bit higher, but for large areas, we can sacrifice with the high efficiency factor, and buy larger size solar panel with lower efficiency and of course it will be price cheaper.
Also, the shadow is another factor in indicating the solar panel size, as for areas subjected to much shadow, we need to use small size with high efficiency solar panels to substitute for much areas loosed due to shadow.
For residential solar applications, typically the solar panel size is about 39 inch by 65 inch with some simple differences from one solar panel manufacturer to another, also the solar panel average power capacity range is approximately from 250watt up to 380watt. These dimensions and sizes remain approximately the same without changes for decades, but the solar panel output power and efficiency have dramatically changed to the better, it is a rapid competition between solar panel manufacturers about who will be the first to present the higher efficiency with lower size solar panel to the market! Because this is the first crucial factor in selling their solar panels worldwide. Even more, the solar system experts always try to change the design for the entire solar system to reduce gapes between panels, using invisible frames, and many other ideas to achieve highest efficiency in case of limited space roofs.
Also, the number of used solar panels is an important factor to achieve required energy production over one day or even over one month/year.
Generally, after indicating the approximate number of solar panels installed in your system and the size/power of each panel, the solar system installer will indicate the best solar array angle to the sun which is a very important factor as well in achieving required energy production.
There are many different solar panels sizes in the market with different output power capacities and efficiencies, but selecting the best suitable one for our application will be the decision of your proficient solar system installer.