There are many different thoughts on this. If you are writing a novel, then you can do basically whatever you want (and I'll get to that later), but if it's an academic paper, you should be more rigid with your length.
For academic papers, your assumption was right-- 5-6 sentences is a good length for that paper. You won't want standalone sentences that you sometimes see in novels, that's just not very on-genre. That part is pretty basic.
A novel, however, is something different entirely.
You are able to do many standalone sentences (like I just did above, look at that!), and you have the option of having much longer or shorter paragraphs. The Hobbit, for example, has many long paragraphs. But that was written a while ago, and long paragraphs tend to draw the reader out of the book. Long paragraphs look boring and dull, making the reader step away from the book for long periods of time. So you probably don't want those large paragraphs (more than ~12 sentences). That being said, when you are catching your reader up on some history that is new for the reader and the main character, then long paragraphs can be fine in small portions.
An average length for a paragraph in a novel is anywhere from 1-8(ish) sentences, so that should be an average guide for you. Your style could be that of long rambling sentences,s which is fine, but if that is you, try to cut back on them a bit.
Don't forget to change paragraphs when the voice changes, time changes, something new is introduced, a setting changes, the "camera" moves, or something new is happening.
This link is one that explains this topic in more detail.
I hope that helped and happy writing!