I’ve done some research in the last years because I was thinking about doing a web-comic too. I asked people who are already publishing for advice and also looked at many web-comics out there how they are doing it. First I found there are basically two types of comics. Web-comics and “web published comics”.
Web published comics
These are comics that happen to be readable on the web but are not designed for it. They follow the usual rules of printed comics when it comes to aspect ratio and size because they are designed with printing as priority.
So, if getting your stuff printed is your main priority, then go with whatever the standard is in your country or your print service provider can deliver (or your publisher recommends). Later you can decide how to present it online.
Web-Comics have become a very different experience than the normal printed one. They often use the features of the web and the fact that you have a page with infinite length (the screen) to aid in storytelling or dramaturgy.
Take this xkcd comic for example. It is one very long panel. As you scroll down you find new things that were outside the screen before and you can explore the scene. Hiding things outside of the screen for dramatic effects is very common. The printed equivalent is to build some tension but for the relief you have to flip the page, so you don’t get accidentally spoiled when your eyes scan ahead. Sometimes the infinite panels is used to show progress over time. As you scroll down new things enter the screens while others leave, breaking the very idea of a panel as the screen itself becomes your panel.
I’ve seen a lot of web comics that add a few empty rows between panels to get that effect, or they use hidden panels like in this one from Mr. Lovenstein. Go on and move your mouse over empty space at the bottom.
Some web-comics even use web features for interactive storytelling like this exocomic.
Since the web is very loose when it comes to formats you can basically go as wild as you want but I noticed there are some unwritten rules like “rather long than wide”. Scrolling up/down feels natural. Scrolling sideways not and feels unintuitive, at least on a desktop. Therefore most web comics tend to get longer than wide. Often not more than two panel columns. I noticed the width did increase for a while as widescreens became more common, so you can fit more columns on a screen but now with mobile devices being common and most people use them in vertical orientation, columns have become lesser again to make it more readable on mobile devices, even having just a single column like an old school comic strip but in vertical. Image resolutions are usually multiples/fractions of common screen resolutions to make the page perfectly fit the screen when viewed in full screen mode.
So if you plan to do a web-comic that is primarily web-focused you basically enter a new world of comic making with a lot of possibilities. However as you go deeper into the rabbit hole, the harder it will get to print your stuff.
The craziest I’ve seen was that every panel was a single image and they rearranged depending on the available space so it was single column on a mobile device in vertical mode, two columns in horizontal and a lot of columns on a desktop. Unfortunately I can’t remember which comic it was so I can’t link this one.