Changing post width

But the width you choose there will be the same for both the homepage

This is how the theme is designed. Values entered on the options page are used to calculate the entire width too.

If you really want to, you could set the width on Frontier Options to affect the single posts then use CSS to target the ones that are only on the home page.

This is just an example:

body.home #content {
   max-width: xxxpx;
   width: xxxpx;
}

You may have to adjust the sidebar for the homepage too

body.home #sidebar-left {
   max-width: xxxpx;
   width: xxxpx;
}

Since the theme is not really designed to work this way, these changes might produce some unwanted issues.