My name is Alex, I'm a guru here at Shopify!
No problem at all! We've got a guide on adding sizing charts to your products that can walk you through how to add those on. Making sure that they only appear on certain product pages takes a few additional steps, but that's totally doable as well!
In Shopify, all products pages are created from a single template - this keeps you from having to design a new page for every single product you have in your store. If you need to make changes to the coding that will only apply to certain items, however, you can create a new template for those products! We've a great guide on creating those templates, and once you have created your new template, you can follow the guide to add the size chart. Make the changes only in the new template that you've created, not the original product template!
Totally understand if you're concerned about making coding changes. If you'd like to make sure that your site isn't affected while you work on this, you can back up your theme to ensure that your website isn't affected as you work - and if you'd rather not do the coding, you could always check out our listing of third party experts for hire for help in getting that function created!
Hope that's helped! Please feel free to give us a call or start a live chat at any time, we're open 24/7 for your convenience and always happy to assist!
Alex | Shopify Guru