Every team has their own preferences when it comes to formatting, but I personally prefer using single-line formatting for CSS. Here's why:
When I'm working on CSS maintenance, I find it easier to quickly identify and modify existing selectors if each line represents a new selector instead of more style declarations. This saves me time scrolling through the document to locate what I need.
Once I've located the selector I want to edit, syntax highlighting helps me easily read the rules associated with that specific selector, maintaining readability even with all style declarations on one line.
While single-line formatting may not directly impact file size, I choose to rely on tools like YUI Compressor for automatic CSS compression instead of manually optimizing my style sheets for efficiency.