Text files used by Notepad should be no larger than 45K. Notepad cannot open a file that exceeds 54K in size which is small and does not allow you to continue editing a file if the file size reaches between 45K and 54K. This is an old legacy program that I do not think has changed since the 80’s. With all the other options available they probably saw no need to expand its function.
Try Wordpad included with Windows instead as far as I know the only limit is amount of memory in your computer less what the OS takes up. So 4 gig of RAM less the OS overhead may only leave about 3 gig for a file in memory.