How do I add a Size Chart to my products?
Reformation comes with the ability to display a sizing guide for your products:

This can be set up inside the Variant Picker block inside your Product Information section:
You need to add a Sizing Guide label and select a page to be displayed inside the lightbox.

I cannot see the Sizing guide link
This is probably because of your Variant name. We have configured it to work with the Variant name, “Size”:

However, this can be edited inside sections/main-product.liquid file. Please find the below code:
{%- if option_slug == 'size' -%}
and replace “size” with your variant slug.