Think of paragraphs the way you think of sentences. You don't want a page full of either very long or very short, or all mid-length come to that. Vary them in size as much as possible.
The other thing to remember is that the opening and closing words of a sentence have more power than the middle, and the same goes for paragraphs. If you want something to stand out and be noticed, it's self-defeating to shove it in the middle of long exposition -- actually it's usually self-defeating to have long exposition at all. I'm convinced people read less carefully nowadays, and the longer a paragraph, the less likely they are to read it in full or take useful information from it.
There are no hard and fast rules. I was going to say definitely never have dialogue from two or more people in the same paragraph, but actually if you're writing a confused melee of voices all clamouring for attention, that might work better with several in one para rather than all in separate lines.
I use dialogue at the beginning and end of narrative, though more usually the former, and I've occasionally used it in the middle, but only where the narrative either side is relatively short, each perhaps only one or two medium length sentences, and where the dialogue itself isn't long. Personally I would avoid a mix of long narrative and speech no matter how constituted. In the example you use, taking the actual single sentences as representing several in reality, I think it would be far better to use three separate paragraphs.
This is quite a long narrative passage about him. How long is long is the question. Three or four medium length sentences should be fine, but the more words, the more your readers will be wanting a new para by now anyway.
He said, "This and that, and the other" before walking out. NB Walking out would usually trigger a new para for me quite apart from the dialogue issue.
He then did something else that needed quite a lot of text to describe. Again if this is a long para you'll be exhausting your readers' patience if it were all on its own, and by lumping it together with an earlier longer one you risk them skipping it in full. Plus if he's doing something else in a different place, this to me would again trigger a new para regardless of intervening dialogue.
If you're worried whether it works in reality, put some examples up in Critiques and see what reaction you get.