Sure, I can answer, although you might not like it. XD The format that's best is the format that the company wants you to use. And everyone has a different format. If you're trying to do a regular comic that you hope to sell to a company like Marvel or DC, they use standard sizes and you can buy comic board/paper in that size. If you're thinking about something more manga-like, or an indie comic, then the company probably has its own size.
The good thing about that is that you don't have to worry about it before you sign a contract with them: you can submit a story or sample work in whatever size you like and they'll give you the technical specs to draw the story after they hire you. It's pretty much a fact of life that you'll be rewriting and re-drawing whatever you submit if you're hired. So don't worry too much about the specs.
The one case in which you *do* have to worry about the specs before you're hired is if the company is running a contest or soliciting already-created material. And in those cases, the company will put the specs they're looking for in the contest rules or the submission guidelines, so you'll know what they are.
Or, perhaps, you decide you want to self-publish. In *that* case, you select a printer first and talk to them about their specs. The company I used to print the comic I used for this tutorial is no longer in the business, so I switched. The new company I found has slightly different specs, but not too different. But I found out what they were before settling in to draw the final pencils of the next comic for them.