How to make my website fit any screen size?

This should work for you:

body {
    margin: 0;
    width: 100vw;
    height: 100vh;
}

div {
    width: 100vw;
    height: 100vh;
}

vw stands for viewport-width, that means 100% of what the user sees (in width). vh stands for viewport-height, the same thing but in height