Oh! This is what I have learned Some of it will be already told by the others, but here it is anyway~
1) Every page is your universe, and so, every panel occupies space in your universe. As you may know, space and time are intimately related, so, the more the space, the more time you use from your page (universe). A little square may be percieved as a little amount of seconds, and so, just little bubble speechs can be logically there, while a big horizontal panel should have more bubbles. 2, or 3. This is the basic rule.
How to create a rythm with this basic knowledge is up to you. You can do 3 squares, and 2 big horizontal; Separate an horizontal panel for a couple of square panel; have some vertical that breaks the horizontal reading, etc.
2) Also, the panels alone have an impact imbued withim them. How's that? Well, an splash page, where only 1 draw fills the page has a more impact that a little square panel. The same with a slash-double-page panel. BUT that doesn't mean that, as they uses so much of your universe space will use that much time. The impact of the panel will brake the space-time rule. So, for more important scenes, scenes where you are situating your characters on set, where you present an important character, where something important will be said, it will have to be a more impacting panel -no little squares here; horizontal, verticals and splash, and it depends to the writter if the impact will be enough to brake the time rule, or keep it.
Lastly, there are a lot of more panels to play, because you are not constrained as a movie, with an square. You can do circle panels, star panels, triangle panels, characters that are out of panel, etc. Everything that breaks the rule number 1, will have a more impact, though.
That's all the information I know.
n_n!