I'm displaying my app within a dialog. The dialog doesn't cover the full screenwith (at least 320dip). Therefore, using AdMob is a bit of a problem, because all AdSizes I've seen take up at least 320dip width. So my question is: is there a possibility to still use AdMob, but with a ad less than 320dip wide (about 280dip or so).
Short answer No. Long answer is you are probably looking at this wrong.
But what do you mean "I'm display my app within a dialog"?
If you have applied a dialog style to your app or are even dispalying all of your UI within a dialog (weird) then you can always configure a dialog to have any width. So make it 320 dip.