There is no option in the gl windows nor ingame.

That because glh2 is setup to work through nGlide wrapper by default.

Remove wrapper-related files from the game directory, namely:
3DfxSpl.dll
3DfxSpl2.dll
3DfxSpl3.dll
glide.dll
glide2x.dll
glide3x.dll
opengl32.dll

And then run the game like this:
glh2.exe -windows <desired_horizontal_resolution_in_pixels>
or like this for software renderer:
h2.exe -windows <desired_horizontal_resolution_in_pixels>

Just fix your shortcut to the desired values.

That might or might not work well on the modern M$ "Windows as Service", though. So, you are on your own to try and check. And do backups before experimenting.