There is no maximum size. API suitability is determined by response speed. If an API takes a minute to respond, we will not be able to retrieve the content fast enough to then build and serve the image live into the email. In this case we would have to wait 1 minute to get the content and then we would build and serve the image in under 400ms. In reality a subscriber would not wait this long to see the image in the email. Therefore the API response should be as quick as possible - our ideal scenario is a response time that is under 400ms. In reality we will time out after 1 second and serve a default image from our cache (image built from the last successful call) into the email.