I assume by "vertical comic," you mean "vertical scroll comic" like most Korean webtoons.
If you're doing a vertical scroll comic, you may want to use multiple A4 papers for a single episode. Even a very tall single panel could require two A4 papers!
There are no ideal panel sizes. Feel free to make them as big or as small as you want them to be. Just make sure all the important bits are visible/legible, and do NOT assign panel sizes randomly. Generally speaking, the more important the moment, the bigger the panel. The longer the moment, the bigger the panel. You don't have to follow that 100% of the time, but it's something to keep in mind.
