I do my comics in Clip Studio Paint as well! I do them in a big canvas 800WidthX6000Height pixels approx (350 Dpi) Although I have tried other layouts. Please not that it's always better to go too big! Rather than to go too small! You can always shrink a big canvas, but you can't enlarge a small one without pixelate it... unless you do high res? Although there is the option of drawing with vector images, but while it might look alright. I don't exactly recommend it... Although it's a option.
Also there are lots of tutorials on youtube for this stuff! I might reccomend some if you want me to?
Also FYI I can also recommend a good cropping website service!