Hello,
We apologize for the issue you are facing with the wrong image size for social sharing in your Custom Post Type. It seems that Rank Math is defaulting to a smaller image instead of the full-sized featured image.
To address this, you can ensure that the Open Graph image settings are correctly configured. Here are a few steps you can take:
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Set the Open Graph Image: Make sure you have set the Open Graph image for your custom post type. You can do this by navigating to the social tab in the Rank Math meta box for your custom post type and uploading the desired image.

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Check Image Dimensions: Ensure that the image you are using meets the recommended dimensions of 1200px X 630px. If the image is too small or not in a supported format (JPEG, PNG, GIF), it may not display correctly on social media platforms.

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Fallback Settings: If you want to set a default Open Graph thumbnail for all posts, including your custom post types, go to Rank Math SEO → Titles & Meta → Global → OpenGraph Thumbnail and upload your desired default image there.
If these steps do not resolve the issue, it may require further investigation specific to your custom post type setup. In that case, we recommend replying to this ticket with your site URL so that a human agent from our support team can assist you further.
Thank you for raising this concern! We hope this helps you get the correct image displayed for social sharing.
Sources:
Open Graph/Social Preview Image Is Not Displayed » Rank Math
Open Graph Meta Tags » Rank Math
This is an automated reply from Rank Math Support Bot. Your ticket has been assigned to a human agent and we will get back to you shortly.
Hi,
all this was checked, the right image is set, but rankmath chooses 300×300.