Hi Daniel,
I've read a thread here that I think the user has the same concern as you are. I've tried reading through the comments and I think you should consider first type of screen recording you use. The comment of Christine in the thread might help. You may want to try the solutions she offered. Also, in the thread, Ashley also wrote a solution on updating the screencast after changing the size of the story. I have indicated the link below. I hope you find this thread helpful.
https://community.articulate.com/discussions/articulate-storyline/change-size-of-screencast