Someone may have already said this but writing a short story and writing a long-form narrative is a not the same. In other words, they are too different skills. You have to build them up separately.
Now if you simply trying to control your page count and story length then we are in a similar 'boat' because I struggle to keep my stories short. This is not necessarily something you should avoid, it can be a very useful instinct however what I have been doing recently to control this instinct is carefully breaking down every moment in my story.
I begin with the overall synopsis, at this stage I don't worry about the length of the story. It's best just to 'let it all out' and spare no detail in the synopsis. Then I decide how much of the synopsis makes sense and which parts need to be told to the audience and at what time for maximum conflict an tension.
Then, specifically for comics, I separate the synopsis into chapters and then separate those chapters into 'Issues' like the classic comics you see in brick and mortar shops.
Finally I break the Issues down into page descriptions. Yes, I literally record what will happen on each page, what the purpose of the page is and what I am trying to communicate.
It's then that I transfer all of that into a comic script. If you need help with creating a comic script then go here.
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This is a method that I have adopted very recently and it's help me stay in control of my narratives no matter what length they are at and it allows me a lot of flexibility in shortening or lengthening my stories.