Chrome and Chrome Beta suddenly won't fill up the entire screen on launch
This is happening in any custom ROM that supports floating windows. It is there, for example, in SlimKat, Spirit ROM, CM based ROMs etc. In my case I can move and size the window, and there is a maximize button available. It resizes the window with pinch to zoom automatically rendering it almost unusable. 

This is an app issue ultimately, but it is specific to custom ROMs with this functionality. 

The only thing you can do it, open the Chrome window then bring up the task manager and select Chrome and it will fill the screen as normal. This is obviously an awkward work around.

Alternatively if you have Xposed installed you can try to Blacklist Chrome from xFloatingWindow and see if it will work. Some, but not all have had success with this.