Re: Main page - auto fit
What you build your site on has no bearing on how it will show to others. What WILL matter is how wide you build your site.
If you build your site between 800 wide then you will have no sidewards scroll whatever PC anyone views your site at, however as a small minority are now still using 800 wide to view sites, it is fairly safe and common to build to around 940 wide.
Start your site as far to the left of the builder as possible and set your page guide borders to the width you want the page to be.
In your page properties set it to 'centre in browser'
Don't aim for success if you want it; just do what you love and believe in, and it will come naturally.