While playing Devinity (in the Sewers of Verdistis, I'm pretty sure that's unimportant, but just in case), the game crashed for the first time ever and afterwards it wouldn't start up without demanding a new configuration. I accidentally chose the wrong resolution because I carelessly clicked okay, thinking the last one would be on default, but now I'm stuck with a window the size of this post and the configuration tool won't adjust the resolution no matter what I try.
Could somebody help me out with that?

For user defined sizes, read Porkdish's post above.

If you are using one of the standard resolutions for the game (640 x 480, 800 x 600, 1024 x 768, 1280 x 1024) you can go to the Programs menu, GOG.com, Divine Divinity and click on the Configuration icon to set one as a default. You can also go to the install directory and run the configtool.exe to manually run the same program as the Program menu icon.