Board Thread:Support Requests - Designing Your Wikia/@comment-28398720-20160814084529/@comment-26402117-20160828155250

Its possibly because images smaller than 200px will have details that are much less visible to users, while images that are larger than 300px could affect the pageflow of the article and damage the reader experience.

Interestingly, the default wiki wordmark is also exactly 250px wide. :D