I am given these default square logos with the first initial of a site name and I would much rather replace it with the company logo which comes to the size of 400x 128. It lets me upload the company logo but even if my company logo is 400 x 128 it still gets truncated to 128 by 128 just the same, making it unrecognizable and futile. Without content editors WP , how is this accomplished? Thanks in advance.
As i understand you are talking about the Modern page of Team or Communication site.
Option 1: You have to find or create a company logo which actually suits the Square placeHolder given by Microsoft for Logo. hope this you might not want to do.
Option 2 : You have to use SPFx extension to customize the Header https://docs.microsoft.com/en-us/sharepoint/dev/spfx/extensions/overview-extensions
Hope these information resolve your concern