On Twitter, Google’s John Mueller confirmed that the size of a CSS file has no influence on the rankings of a website:
The amount of CSS is usually no problem (<10's of MBs), for meta-tags we just need to be able to find them in the head.— John ☆.o(≧▽≦)o.☆ (@JohnMu) 7. August 2017
Google can handle very large files and the size of a CSS file is usually not a problem for Google.
How to check the size of your files
Among many other things, the website audit tool in SEOprofiler checks the size of your website files. You also get information about the speed and the performance of your files. If you haven’t done it yet, check your website now with the website audit tool in SEOprofiler: