Storyboard images must be at a reasonable size for their usage. For most uses (e.g. full-screen images), the maximum is 800x600 pixels, although 640x480 is highly preferred. The editor is 640x480 at default, so there's really no need to use anything larger for full-screen purposes.I'd like to suggest:
The maximum dimensions for tall or wide scrolling images are 640x1440 and 1920x480 respectively. This is for the sake of gameplay performance, image loading time, and filesize.
For now, Move these to guideline, since the storyboarders can split the big images while using more SB codes, and the performance will not be changed. Technically the rule rules nothing. (And file size limit and SB load limit are enough here IMO)
In the future, when it is technically fine, change the dimension limits to a proper amount, since we have larger BG size now and this seems outdated.