When frequently working in a program like Photoshop® or Gimp® for web design purposes, you’ll soon find yourself becoming familiar with the distance 600px (pixels) reaches in width or height (as compared to your own computer monitor anyway).
This becomes helpful in at least two ways.
- Guestimating element sizes. When you want to determine a good size for an image, box or some other element, you can often get it right or very close in the first try. This comes with practice.
So although it can seem difficult to have an intuition with pixels, it is very useful and actually tends to come naturally over time.