Google's John Mueller said on Twitter that the size of your CSS file generally doesn't have a negative impact on your rankings in Google. He said Google can handle "<10's of MBs" and that "the amount of CSS is usually no problem."

Of course, if your CSS is a novel, Google will probably stop trying to crawl and render it at some point. But here, John Mueller is giving you an approximate number of 10s of megabytes to stay within for your CSS file size limits.

Here are the tweets:

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) August 7, 2017

There are always exceptions to the rule but keep this in mind.

Forum discussion at Twitter.