I already posted the reason for choosing SD Card. Windows will not work with Raspberry Pi. Operating system is working on Hardware. So hardware must be compatible with Operating System. We can only use OS customized for Raspberry Pi
Your questions are going outside the scope of this topic. You can open a new topic for asking such questions.
Size depends on the Operating System you are using. You can check sizes of different operating systems. You may customize it to reduce size by removing unwanted features. If you want to customize Linux, please go to a Linux forum.