Thank you for your message, I would be happy to offer you some suggestions for this. You have two potential options when creating a sizing guide for your product pages, I would be happy to outline them both for you. You may be able to follow the tutorial in our help guide here, for adding a size chart to your products. However, if you can’t follow this tutorial for any reason, you will need a developer to make the changes, and then you would need to contact that developer any time you have a question about it or want to change it. Alternatively, if you are using a Shopify theme, our themes support team may be able to help you with this customization. However, in all honesty, I would advise simply adding a free, third-party application like Size matters to your shop. The app allows fully customizable and unlimited size guides for your store, there may be some set-up that is required, however, you could always reach out to the apps developer-support with any questions:
I hope this gets you on the right track, please feel free to reach back out here directly with any questions you may have!
To clarify, you should be able to add the size options from the app anywhere you are showcasing products. It may be worth reaching out to the developers of the app with any questions you have, you can reach out to them via their support page here.
I hope this helps you out!