I tried some more settings and got odd results:
I turned off the “use single process” option. When I play as client or listen server (in option: editor multiplayer mode) and than test as standalone game I get the window size i specified in “multiplayer window size (in pixels)”. If I choose “play offline” (in option: editor multiplayer mode), the play window, as soon as I test as standalone game, stretches over the entire width of one of my monitors…