You can add different size charts for different products.
To edit Theme Settings of your site, Navigate to Shopify (Admin Panel -> Online Store -> Themes -> Customize).
1. To add a size chart for all products, navigate to a product page while customizing the theme. Open "product" settings under sections. Scroll down to Size Chart image, and add your size chart here. You can also add a size chart using custom tab content, and creating the size chart using Shopify's page editor.
2. To add different size charts for different products, upload the size chart to product images (Admin Panel -> Products -> Select the product). Edit the alt text, and add text "size chart" to configure the image to a size chart tab for this product.