I have found the 5" Libre screen to be great for reading just about everything but;
1. books with lots of images
2. PDFs (some work great with reflow but most of mine (just free stuff) don't work well)
3. magazines from google-books
Even newspapers (from Calibre news-feed) work great (when they have a TOC).
6" I imagine would be more than adequate for everything but the above mentioned (which IMHO would require a much larger screen. Say 9"?