If you want to be thinking about printing (which I think is wise if you'd like to print someday!), you can look at companies that print short-run or print-on-demand comics and see what sizes they use! I used Ka-Blam's dimensions when I was first getting started (I used Standard Sized Comic page dimensions, I think), though I know there are others -- just look up their technical specifications or page dimensions, they'll usually have a couple different standards to choose from, and you can see what fits your style best!
Basically, it's not just size that's important, but proportions as well. Image you drew your comic in the shape of a square, and then you wanted to print it. Most smaller book-printing companies won't do custom sizes (or they'll charge extra for them), so if you print short, square-shaped comics in a rectangular book, it'll have a lot of extra space and look weird.
The same goes for if you make your comic rectangular but not quite the same rectangle that the printer uses. Like, if they print comics at 6x9 and you've been drawing your comic at 6x12, you're going to have to shrink down your comic a lot to get it to fit, and it might not look good. So it's good to pay attention to what proportions printers use, if you want to print your comic at some point!