If you have a more detailed style I would definitely recommend doubling the pixel ratio! I personally found 800 wide to be very difficult to get thin lines with. Especially if you have a lot of perspective shots, or anything in the background.
The webtoon canvas has updated already :) the 800px must be fixed, but you can adjust the length , webtoon has automatic slicing function. For example: you uploaded a picture of 800x10000px, webtoon will automatically slice it once you uploaded the file. You can try it with your drafts to see how it works ^^
You can use a larger canvas but I would suggest to keep the width to 800pixel size. For the length you can do it until 10,000 pixel size. Going max will slow down your app
At that time, the max px size of Ibispaint is 4096. It’s better to work on a bigger/longer canvas size so you can have a better pacing of the panels in your story :))