Are you pressing the shift-key? Alone, the key produces "=" while with shift produces "+" on my keyboard. I used the =/+ key next to backspace.

Also, the game don't respond to the "+" on the numpad, it respond on the +/- keys next to the backspace.

I see. That may just be why it didn't work for me then - My + key isn't next to backspace and the plus sign is the "default" button, not with shift. It is indeed the swedish keyboard that messed it up. It's common for me to have problems like that with older games, as they often want you to use "the bracket keys" for instance, where they are only shift functions on the number keys for me and I have to try the entire keyboard to see if "Ä" will trigger something.

It worked out anyway though, cause I found a slider in the game menu for "screen size" which did the exact same thing, and got it back to normal :)

Glad you got it sorted. Not the first time this kind of issue has cropped up with old FPS games, and it appears to have been the exact same cause.

You might want to try out some source ports where you can rebind all of the game controls, so that your language keyboard works for all of them. Chocolate Doom is for a vanilla DOOM gameplay (though I've not tried this port). Then there are ZDoom and GZDoom (an OpenGL offshoot - my current port of choice) that can be tweaked for better graphics and extra functions for newer mods like jumping and crouching, kicking, etc. And Zandronum is a multiplayer focused port. I used to use the Doomsday Engine which is designed for very pretty graphics and lights, but I moved away from it in favour of GZDoom.

My laptop has a similar problem; dosbox also doesn't recognise the right shift button. I think it's something to do with the keyboard drivers, but as I rarely play games on it (prefer to use a desktop), I have never bothered to work out how to resolve this.

You can change the keyboard mappings for DOSBox by pressing ctrl+shift+f1
You can then reassign any key within the DOSBox emulator.