Have you got Safari set as your home page? If not, set up a Safari screen as you would like to see it each time you open. Go to Safari preferences > check the home page url is the same as the page you set up (it will be in yellow) > click on 'set to current page' and from then onwards, that will be the page and the size of page that will load for you.
It doesn't work.
This is how i want it every time when i start safari. Fullscreen mode:
This is how it starts now.. (WRONG windowed mode)
And this is my settings
Open safari: New window
Open new window with: Website
Open new Tab with: Top sites
Sorry to read that. I do not have 10 8 Mountain Lion so it may differ, but my settings are ...
Open with .... New Window
Open new window with .... Home page.
New tabs open with .... Home page
Home page .... In my case Safari BBC. Which I had set the page to.
(I am assuming Home page for me equals Start page for you).