How big can i make my css file ?

I don't think there's a hard limit, but there are practical limits. Two of the most important practical limits are developer sanity and performance.

One giant CSS file is a pain to maintain for developers, but it's better from a performance perspective because the browser is only sending one request for the CSS file rather than several requests for different CSS files.

A lot of developers use a CSS pre-compiler like Sass or LESS to get around this. They let you split CSS up into multiple files (often called partials) and the pre-compiler assembles them into one CSS file.

The benefit is that it's a heck of a lot easier to find what you need to edit in the smaller partial files while still getting the performance gains from loading one CSS file.

When they're ready to move the project to a production server, developers will also minify their CSS. Minifying CSS compresses the CSS by taking out all the unnecessary spaces, new lines, and and comments without actually affecting the functionality of the CSS.

The browser can read the minified CSS just fine, but it's a lot harder for humans to read because all of the formatting is removed.

Here's a CSS minifier you can use to see what it does: Dan's Tools CSS Minifier

So the current best practices are to maintain a set of multiple partial files for development, but to merge them into one CSS file and minify it for the production environment. That way you keep the developers sane(r) and still get the performance from reducing the number of browser requests.

Roy