Add the size chart to the standard product template.

//setting

{
“type”: “page”,
“id”: “size_chart_page”,
“label”: “Size chart page”,
“info”: “Feature a page for size option”
}

//show

{%- if block.settings.size_chart_page != blank -%}
{%- assign page_handle = block.settings.size_chart_page -%}
{%- assign page = pages[page_handle] -%}
{%- if page -%}
Size Chart
{%- endif -%}
{%- endif -%}