Try different sizes and see what you are more comfortable with. You need enough resolution for the details you are sculpting, but not much more (can cause slower viewport and even crashes). Relative Detail uses the screen pixels for the size of the edges. A Relative Detail of 7px will be coarser on a 1920×1080 monitor than on a 4K monitor (unless I am mistaken).
Anyway, the intuition will come with experience (practice, practice, practice…).
