How to adjust text's width and put it at center of post (Visual) editor?

How to adjust text width and put at center of post (Visual)...

1. Make the Visual editor occupy the whole screen. (I use the classic editor.) ...
2. Set a width for the text (because currently text is almost from edge to edge of screen)
3. Put text at the center (but “aligned left,” not “justified”). This is for the “left and right margins,” within the post editor.