Dear Rahul,
It seems that you can’t use the modern SharePoint experience in your environment. For the requirement, you may need to add the CSS code in the Script Editor web part on your site page. I find a code which may be applied to your environment for your reference.
You need to first click on Edit page to see whether you can add the Script Editor web part. If no, you may need to contact your administrator and follow the steps in the article to enable custom script in SharePoint Online Management Shell first.
After running the command, refresh the page and you should be able to add the web part. Then insert the following code in the web part to see the result.
<style type="text/css">
.ms-vb2 .ms-rtestate-field {
max-width: 100px;
max-height: 380px;
overflow: hidden;
}
.ms-vb2 .ms-rtestate-field:hover {
max-height: none;
}
</style>
<h2>
</h2>
Before:
After:
Best Regards,
Cliff