how can i better understand px, better understand what detail size i should be using when using dynamic topology. How can i get more intuitive with knowing what size i should be using?
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 1920x1080 monitor than on a 4K monitor (unless I am mistaken).
Anyway, the intuition will come with experience (practice, practice, practice...).
Practice x 3
Yeah, it all depends on your judgment and it depends on the detail you need and you need to be the judge of that. Where do you get that judgment? The same old answer, practice. Sorry, maybe not the answer you were expecting, but that's how you get the understanding you're after. Do not despair, if you keep at it your brain will click and you wont even notice.