By default, the Kiwi Sizing script is loaded on every page on Shopify. The script is loaded asynchronously, so it shouldn't hinder your site's performance, but it could potentially lower the performance rating from some evaluation tools.
If you want to control specifically where our scripts are loaded, you can first disable our default script loading.
Go to Settings > Advance > Others, and uncheck "Disable default JS script injection on all pages."
Now that you do this, no scripts from Kiwi are in your shop, and you would need to add them manually into your theme.
If you only want to display our script on product pages, you can open the theme's code editor and the "theme.liquid" file.
Then add the following code right after </body> tag.
{% assign t = template | prepend: '/' | append: '.' %}
{% if t contains '/product.' %}
<scriptsrc="https://cdn.static.kiwisizing.com/SizingPlugin.prod.js"></script>
{% endif %}