How do you determine what sizes/margin/padding to use?

Hey, everyone! This is a very general question, but one that bothers me constantly: unless you're in a situation where a designer has handed you a PSD and you can inspect it to see what spacing and sizes they've used for everything, how do you determine what padding and margin to use on each individual item, and what size everything should be?

Is it as simple as playing around until you like how it looks? Surely there is a better, more precise way. I feel as if I waste all of my time on this and never get into the more fun parts of CSS. I keep playing around and guessing at font sizes that will look good, or adding/removing margins here and there and padding more padding here and there to see if it gives me the look I want. It feels overwhelming and I feel quite stupid that I am hung up on something like this. I would appreciate any advice!