Basically it would be enough to add a new scene template with the main layer pre-set to 1024x768. It can be done easily:
1) go to <WME directory>\templates\scene
2) copy the entire "Empty scene 800x600" directory to a new directory (for example "Empty scene 1024x768")
3) inside the new directory, edit the "$$NAME$$.scene" file in some text editor, and replace all occurences of 800 by 1024 and all occurences of 600 to 768
4) additionally you can edit the _template file and change the $DESC (description) line appropriately
As for turn-based combat, I'm afraid there's no simple answer to this question, because "turn-based combat" is a very vague term, and it can be implemented in many different ways. So, it's certainly doable, but the amount of scripting can vary depending on the complexity of your combat system.