A panel is like a sentence.
Sentences can be formal, informal; correct, incorrect; effective, flimsy; charged, average; a question, a statement, a command. The main thing sentences have in common? They are separated ideas. (Note the incorrect sentence that is a question, for effect.)
Panels are like sentences. They should express one main idea clearly. They can be complex, simple, full, or empty. They can be anything you want them to be! However, they should still help convey one idea.
It was more common in older comics to attempt to express a LOT in one panel:
While this does all fit in this panel, it might have been cooler to leave it more to the imagination, so you could see the impact on the poor sap who was to wear the specially-treated monocle.
That "mistake" is much more rare today. We tend to put too little in one panel.
This one is okay. It's right on the edge of being an appropriately dramatic, short "sentence." The context defines it as a sudden impact, so you're allowed to break it up. (The problem is that all the panels are this same "length," with no variation.)
This one isn't a special moment in the comic. It's just this way because all the panels are, again, this short "length." If they were sentences, every one would be about four words long. Wouldn't that get annoying?
"The fury of this audience is terrible. This audience. This last court of appeal. This audience in its fury is terrible. ...
"The audience is pulling up the benches. A tomato shatters itself on the Prince's right eye. An over-ripe tomato.
...Three eggs and a cat sail through the air. Falling short, they drop on to the orchestra. These eggs! This cat! They fall on the conductor and the second trombone. They fall like the gentle dew from Heaven upon the place beneath. That cat! Those eggs!From THE SWOOP! Or How Clarence Saved England, by P.G. Wodehouse. The above passage is intentionally bad, and is taken from a LENGTHY set of paragraphs in the same style. If you want to read it, it is free and legal here. However, be warned: many early-20th-century racial and/or national stereotypes are present, though ostensibly played for laughs. If you want to only read the complete passage, it's at the end of chapter 7.
This is one of the things that Japanese comics do well: breaking up their "sentence length."
Here's one from Beet:
Notice how they let the first panel be long so that when she thought of Beet (who is kind of her OTP and also the main character), it could have a bigger impact. If it were like a set of sentences, it might go something like this:
As she prepared to face her fate, Poala thought of those who mattered most to her: her father and mother, both of whom she would never see again--
At this thought, she remembered all the boyish promises he had made; the careless way he'd said it: "Once I'm a top Buster, I'll marry you!"Variation. It's important. Much more pleasant to read than the gobbledygook in the previous passage.
Comics are like sentences. Using that benchmark, I've found it a lot easier to figure out what to put in my panels and how to organize them better, even pushing me to try and express more in one panel.
Hope that helps!