There are four methods for creating a size chart for your products.

Default #

Display same size chart for all products.

Regardless of whether you choose to display the size chart by brand, type, or metafield, the default size chart will still appear for all other products.

For example, if you select a size chart to be displayed by brand or type for a specific clothing brand or vendor, all other products will revert to the default size chart (which pulls content from the selected page in theme settings).

Step 1: From your shopify store admin -> Click “Online Store” -> Pages -> Click “Add page” to create size chart page.

Step 2: From Theme editor(Customize) -> Theme Settings -> Product Page -> Enable “Show Size Chart Link?” and select page to pulls size chart content.

By Type #

Display size chart by product type. i.e one size chart for women top, one for men shirts another for kids and so on.

If you have few products with product type “Shirt” and you want to display same size chart for all products whose product type is “Shirt“.

Step 1: Create size chart page from store admin, Page title must be “Product Type + Size Chart “.

Step 2: For different product types, Create different pages with different product types and append the key phrase “size chart.”

If you have not created pages for all product type then default size chart will display for rest. Already explain above about default size chart

By Brand #

Display size chart by brand(vendor). i.e one size chart for puma, one for nike another for adidas and so on.

Step 1: Create size chart page from store admin, Page title must be “Brand(vendor) Name + Size Chart “.

Step 2: For different product brands, Create different pages with different product brands and append the key phrase “size chart.”

If you have not created pages for all product brands then default size chart will display for rest. Already explain above about default size chart

Metafields #

Dispay different size chart for each products.

From your shopify store admin –> Settings –> Metafields and metaobjects 
–> Click Products

Click on “Add definition” and add data as below

Name: Size Chart ( you can choose other name)
Namespace and Key: custom.sizechart (MUST)
Content type:Text ( Select Multi-line text )

Now go to your products and Scroll down to “Metafields” and Add data in Size chart field

If you don’t add data here, products will display the size chart, which has been set from Theme Settings -> Product Page -> Size Chart.