Hi Ori -- The three available docked panel size options are the only options at the moment (additionally there is the floating panel layout, which also has three panel sizes).
We can take a look at providing additional flexibility here if there's a strong need for it. Could you please provide more information about why none of the current options fit your needs and share some examples of the types of content and stories you'd create with the other panel width?
Owen Evans
Lead Product Engineer | StoryMaps