Lightning Invoice QR-Code minimum size/display resolution?
First of all yes it is possible to convert all characters to uppercase a trick some users of lightning already use. 

If you look in the lightning rfc you can see that the length of an invoice can become quite large as the discriptions, routing hints and fallback addresses are not fixed I think there is not even an upper limit defined. For example this test invoice has 439 characters.


  lnbc20m1pvjluezpp5qqqsyqcyq5rqwzqfqqqsyqcyq5rqwzqfqqqsyqcyq5rqwzqfqypqhp58yjmdan79s6qqdhdzgynm4zwqd5d7xmw5fk98klysy043l2ahrqsfpp3qjmp7lwpagxun9pygexvgpjdc4jdj85fr9yq20q82gphp2nflc7jtzrcazrra7wwgzxqc8u7754cdlpfrmccae92qgzqvzq2ps8pqqqqqqpqqqqq9qqqvpeuqafqxu92d8lr6fvg0r5gv0heeeqgcrqlnm6jhphu9y00rrhy4grqszsvpcgpy9qqqqqqgqqqqq7qqzqj9n4evl6mr5aj9f58zp6fyjzup6ywn3x6sk8akg5v4tgn2q8g4fhx05wf6juaxu9760yp46454gpg5mtzgerlzezqcqvjnhjh8z3g2qqdhhwkj


QR Codes themselves come at different versions which define how much data can be stored in them depending on how many pixels are used. 

What I can't tell u if one pixel of a display would be sufficient. I guess even with error correction this would not be enough. For example the RaspiBlitz uses a 320 * 480 display