It was said the regions in D:OS are comparable to DD in size, but I don't know if that is due to any limits in the editor, or hard drive / memory requirements or if that was just thought to be large enough to fit everything they wanted to put in a particular area.
The editor does have a GUI (several of the kickstarter updates feature videos of things being done with the editor. There are some things that are automated, the dialog editor has templates for various situations, etc.
Currently you can not use the editor to change any game systems (like switch to real time combat or create new spells). Larian has plans for additional games using this engine, though, so after D:OS is released the editor will be update to facilitate this.
Existing spells can be modified and effects changed.
