Omnibox seems to take a minimum width of 256px, plus the width taken by the back, forward, refresh and menu buttons restrict in my case the window to a minimum width of 390px. Also, every pinned tab you have increment the minimum width by a non-constant measure of pixels. In past versions the number os showing extensions also used to affect, although currently they hide/show automatically on resize. I haven't found more workspace related variables which alter the window minimum width but I will post here as I discover them.