For that you have to design popup for your size chart and place size cart page code in popup and open it from product page.
1. Create popup from this link and place its css in Asset->theme.scss file at bottom and add its Js in Asset->theme.js file at bottom
2. place you size chart code in modal html and place bellow link in Section -> product-template.liquid file where you want link
<button id="myBtn">Size chart</button>
Let me know if you need help