Hi Spyros,
We mean that you needn't to select the category, after create the size guide.
So you can show all size guide list which are created on size guide menu on product admin page.
Please ignore the category option.
https://prnt.sc/w37szj https://prnt.sc/w37tq7
Above screenshot will be helpful for you to understand easily.
We changed the backend code, so you can't see the UI changes,
If you have other questions or didn't understand correctly, please let us know again.
Best Regards, C-Themes
Yes, you are right.
You need to choose the size guide on each product page.
Which logic do you want for this?
Best Regards, C-Themes