Obviously think of what your going to want to be putting in there to get an idea of what size door you want. As your using mdf you wont have o worry about making it to a specific size as you can cut it to fit after.
First like you said construct a suitable size gap for the door using the current frame work that's there. (Remember to take into account you'll need to run a door frame round this gap so you can get a good finish when you decorate it, so make it slightly bigger than what you want it to be, maybe 36mm extra height and width if your using 18mm mdf for the door as you can use it also for the frame) Once you've done this you can fit your plaster board surrounding the door gap all the way to the edge.
I would now cut lengths of mdf to make the final door frame and fit this out to about 3mm more than where your plasterboard sits. (if your going to skim it that is). For example if the frame is 40mm deep, plasterboard is 12mm, you'd make your door frame 55mm deep to cover the wood and edge of the plasterboard. Once the door frame is in you can skim round and up flush. Before then cutting and fitting the door.
Hope this helps
Gob's