Nope, don't go milling around the exe, or even mentioning it here. It kind of pisses off the devs, and violates your EULA.
Anyway, look at defines.lua in the /Crusader Kings II/commons/ folder. This is where you can easily modify a whole bunch of variables made available to us (there is also defines.txt but it only lets you modify starting and ending dates for the game and the age of adulthood).
Okay, open defines.lua with notepad or something similar. It is a text format file, best opened with Notepad++ actually (a freeware txt reader).
Then search for "demesne": now you can modify the base demesne limit, the limit extension per rank (count, duke, king, emperor), the limits based on stewardship (it says intrigue, but it is actually stewardship), and finally since 1.05 points per level of legalism--which means that if your capital has a high legalism tech level, you will get one or two extra provinces.
Demesne limit is a sort of gamey way to force you to hand out land, but you can adjust it as you like. You can also modify how many duchies a king can 'legally' hold, etc. in the same file. Spend some time and play around with it, but remember what you change affects both you and the AI.