It's possible, but the demesne limit is how many holdings you can direcly control before penalties and problems start to happen from being over limit. You are talking about a trouble-free 7 to 15 holdings (or three to six counties depending how you want to look at it).
You may want to keep bishops and mayors as vassals for the widest spread, holding only your 7-10 demesne holdings as county capitals. That can spread you out to about three or four decent duchies. Bishops and mayors tend not to rebel like lords.
You will have to banish everybody if you want to be significantly larger since the demesne-over-limit opinion penalties will encourage everybody you haven't imprisoned yet to rebel against you. I'll believe it when I see it, but I remember someone claim that you still have tons of money and troops even after all those penalties from being massively over your demesne limit.