So I've mainly seen Alan Moore throw around this method for figuring out how many words fit on a page or in a panel. He attributes it to Mort Weisenger, a former editor of DC Comics.
Basically it breaks down like this. He proposes that you can comfortably fit 210 words on a comics page without it looking too text heavy. So if you have a six panel grid you've got 35 words per panel.
I think this is a great tool for writing comics where it's important to say as much as possible with as few words as possible as space is limited and the word/picture ration is important as far as being pleasing to the eye. Another way to put it: If it's too long