Hi,
The best practice for setting the content container with is done through Appearance > Customize > Layout > Container and set your content container width there.
You can also set your content width on Layout Elements which can be found on Appearance > Elements when GP Premium has Elements module enabled.
https://docs.generatepress.com/article/layout-element-overview/
There’s also a Layout Metabox if you want to set the content width per page.
https://docs.generatepress.com/article/layout-metabox-overview/
As for setting widths for specific class selectors, avoid using values that are more than 100%.
It’ll affect how the layout looks especially when you use margins and transformations. I’ve checked your site and found that you have CSS codes that use 220%, and this will surely cause things to look weird.
As for images having half the width of their containers, it’s most likely due to it having a set width of 50% or its display size used coincidentally looks like half the container width.