How do I get posts where image is larger than…?

There is no clean way to do this. Each image stores image information in post meta table, but as a part of the serialized PHP string.

  1. Using MySQL for that is out of the question because MySQL doesn’t have a syntax for extracting regular expressions matched content, it can only use perform true/false/like expressions using regular expressions.

  2. The only other solution is PHP looping through all posts, loading featured image and comparing sizes. Depending on the number of posts with featured images you have, this might very slow and time-consuming, especially if you need to run it in the real time.