So after starting this game up I was quite bothered by the default resolution. So I wondered if there is a way to change it? Usually I can fiddle around with some DosBox files or the GoG setup but Imperalism 2 does not seem to run on Dosbox nor does it have a setup.

Can anybody help me out?

Windows games from that era usually worked on only 1 specific resolution, because graphics hardware was much less powerful than today's chipsets. (back in 1997 few consumers had a video card with hardware acceleration)

You could try if your graphics card has settings to enhance the screen at lower resolutions.