I made you a mod that changes the UI Width: https://steamcommunity.com/sharedfiles/filedetails/?id=1479501214
To change the ui-width after downloading the mod, enter a level, open the console (key to the right of l), then write "ui_width 0.5" to change it to 50% of the window-size.
I haven't tested it extensively, so if you notice anywhere the HUD doesn't get scaled where it should, just tell me and I'll look into it