With regards to your own story: I don’t think there’s an exact “cap”/“length” for short stories, but if you’re adapting a script for a short story webtoon, 9 pages sounds like quite a lot...
Of course, that also depends on what you’re writing too - so it may be that 9 pages could be more than enough for your entry to this contest.
Here’s some things to consider:
Is your story dialogue-heavy?
Does it require explanations of complex concepts?
Does your descriptions for scenes in your comic take up some of the wordcount/pagecount?
When it comes to writing a “good” short story though, I think one thing that can grab people’s attention is by coming up with a simple, but unique premise.
I’m definitely not the only competition enterer that used the ocean as a theme, but I tried to make a one-of-a-kind story by having my character be an unconventional oceanographer who tries to make her way down to the bottom of a trench. It doesn’t have to be super complicated to catch a potential reader’s attention.
Here it is for reference:
As for what makes a good “heartwarming”/“mindbending” premise... I can only speak for “mindbending”, since that’s the theme I’m running with. It’s a pretty vague descriptor - but I think taking a look at the sample comics Webtoon provided for the category can help.
For me personally, I interpreted it as “horror mixed with social commentary” - since one of the examples was “Friday: Forbidden Tales” - which did that all of the time.
I hope this helps!