What would be acceptable size of list for O(n!) algorithm
It depends a bit on the constants hidden in the $O$, so no general answer is possible. In my experience you can aim for the low teens. 13! is about six billion. A computer can do about a billion things in a second, so a runtime of a couple of minutes seems reasonable if you only want to enumerate permutations. 15! is already more than a thousand times bigger, so most likely whatever you want to do will take too long.

On my machine counting to 11! in Python takes 14 seconds and counting to 12! takes two and a half minutes. Counting to 13! would hence take about half an hour. Counting to 15! would take about a week.