Hello, @TheOne,
We are delighted to have you join our support forum and hope you are doing well.
You are correct in that if you set the Size Guide to a single product by editing it, then you must also set the size guide widget in the single product builder in order for it to be displayed. This is the current functionality.
If you would like to change the behavior or add an option to enhance its functionality, please post your request on our taskboard at https://www.8theme.com/taskboard/. If it receives votes from other customers, our development team will attempt to include it in one of our upcoming updates.
Thank you for your understanding.
Best Regards,
8Theme’s Team