How to set up default window size

Hey guys I'm fairly new to linux, and decided to install Ubuntu 15.10 on my laptop alongside Windows 10. For the time being i'm very satisfied with the speed and how everything works the way it should.

As i started customizing my desktop i couldn't figure out how to set the default window size with compizconfig Settings Manager (I'm using Unity as the desktop manager).

Any help is appreciated. thanks.

Sorry, but could you be more specific what you mean when you say window size?

width and height. For example, when I open file explorer I want the window to be 600px by 800px.