Anemone theme has following image sizes,
Anemone Thumbnail: 150 x 150
Anemone Featured: 720 x 540
Anemone Featured Hero: 2000 x 800
Anemone Post Carousel: 750 x 750
We normally recommend to use biggest and sharp post thumbnail as you can. Suppose you upload a big post thumbnail that is larger than Anemone Featured Hero, So it will cover all the available sizes automatically.
For most of our demos, we use minimum of 2400 x 1800 images. We ensure that width should be 2400 and height can vary depending on the image. In this case, these images satisfy the every available size of theme.
Big sizes also support you when you are willing to change the theme. You only need to run Regenerate Thumbnail plugin and that’s all.