I've seen a 24 inch checked size bag go through the scanner, I'm pretty sure I've seen a 26 inch go through. Don't think I've ever seen a 28 inch or larger. It doesn't have to go through the scanner, it could be hand inspected if it's too large for the scanner.
I don't think it's ignoring, just that no one here has an answer for you. The only time I flew standby I had a confirmed seat on a later flight and checked my bag for that flight. My checked bag made it on to the earlier flight with me.