I don't think there's a point like that because there'll always be exceptions, but below 3/8 is almost never good. The best short games on this site like Snow and Deepwoods Flight are still 3/8 and 4/8, respectively. Realistically, it's super hard to compare a long story to a short story. A short story can be great as a fun little action piece or a gruesome horror, but those are completely different to one of the great long ones that gets you emotionally involved with the characters and brings you in. With length, no, I don't think it can be too long to be good. If a story can successfully enrapture me, I can't get enough of it.