On the product display, there are basically 2 columns:
The image, which is grid-14
And the details, which is grid-8
However, I've changed my content width from grid-24 to grid-20, and the two columns no longer fit side by side. I want to change the image to grid-8 and the details to grid-10, but I can't find either a template or a display setting where this is configured...
What template are you referring to? This is not really a Drupal Commerce question, this is a theming question in general. With any theme, you should create a child theme of it, which you can alter the grids, font, anything and it will respect your values. If the columns are not fitting, it may be a margin, padding, width, float, position, etc. error. So its not very easy answer this.
If you are using the Drupal Commerce Kickstart profile, you could create your own child theme of the one they are using and if you look at the display's tpl.php file you should be able to adjust the columns.
