Starting 31st January 2027, Google requires all product images to be at least 500 pixels by 500 pixels. It used to be 200 / 300 pixels, depending on the niche of products.
While Google explains that you will get a warning in Google Merchant Center about which images are flagged for being too small, we all know that is not really the case. Google is very sporadic when it comes to detecting and recommending things.
That is why I built a tool to check it all in 1 go for all products. Have a look here: https://merchantcodex.com/image-size-checker-for-google-merchant-center/
Honestly, I recommend ensuring the images are at least 1500 * 1500 pixels, as that will help you get a higher ranking. This will also help you get an excellent rating for your Trusted Store badge. You can check the current ratings here: https://merchants.google.com/mc/quality