By default iframe does not understand parent page height and that is the reason why you must add height value to iframe style attribute.
But it's possible to use Javascript library Open Iframe Resizer https://lemick.github.io/open-iframe-resizer/ and make iframe to follow parent page height.
Note: Open Iframe Resizer is third party Javascript library and Surveypal doesn't have possibility to update or fix it. If you have problems with it then you can send questions to library owner.
Add Open Iframe Resizer library and custom code to your survey
Go to your survey what you will embed to your website.
Go to Build view, click Settings and select tab Snippets
Add below script tags to your Body bottom section
<script type="module" src="https://cdn.jsdelivr.net/npm/@open-iframe-resizer/core@latest/dist/index.js"></script>
Press Save.
Add survey iframe to your website
Go to survey Distribute view and click Embedded survey and copy iframe code.
Next you need to edit the code a little.
Add id="surveypalIframe" after iframe text and before src attribute.
Remove height:600px; from style attribute.
Now your iframe code should look similar as this:
<iframe id="surveypalIframe" src="https://q.surveypal.com/form?sid=1552416441&sh=lX-GObc8HaZIWrezyzSF9WvBRER1eWUxkxNVq2VZ2o7znqJmoEnbU_XG-O66-kBK&channel=website" style="width:100%;border:none"></iframe>
Add this code to your website where you want to show the survey.
Add Open Iframe Resizer library to your website.
Add below script in the end of your website code before closing body tag (</body>)
<script type="module"> import { initialize } from https://cdn.jsdelivr.net/npm/@open-iframe-resizer/core@latest/dist/index.js"; initialize({targetElementSelector: "form", offsetSize: 260,}, "#surveypalIframe"); </script>
Save your changes and try how your iframe works.