With an Internet platform like this you can do pretty much any format you want, but it is good to think of a ratio that will be comfortable to read.
If you're going for a comic-strip style, while it can be convenient for mobile device to have each panel posted separately so they can fit better on the screen, it is then also good to think of shrinking them so the whole panel fits on the screen (a height of 600~700px should display correctly on pretty much any screen).
If you're going for something more like comic/manga, the question you can ask yourself is how much text you intend to have, and how much detailed or minimalist is your style. In Singmire Haze I use an A5 document in 300dpi, which is much smaller that what pastchaser uses but suits my needs (well, I also stick to this because my computer lags a lot with larger documents ;n; )
Once I'm done I downscale the document to have a 940px width and export it for the web in JPG to reduce the size of the file so it displays more quickly for people with slow connections. This is a tutorial on how to reduce file weight in Photoshop
Don't hesitate to look around to see what other people do, what looks like something you'd do. Time ago I used to take screenshots that I'd crop to figure out a ratio in pixel. When you have the height-width ratio that suits you, just increase the scale of the canvas and you're pretty much set. And make sure to work in 300dpi to have more precision. 72dpi is way enough for the web, and 120dpi is enough for printing, but such resolutions aren't fit for drawing.
