What is the recommended size for the post featured images?

Since the featured images at INK adapt to the dimensions of the browser window, there is no standard size for them. You should find a balance between quality and file size, and this may vary depending on the image. For the demo we use images with dimensions of about 1200 x 800 pixels. 

In any case, if you use large sized images, we recommend an image optimizer web app which is free to use and provides great results:  http://kraken.io
We just like using it ourselves, we are in no way affiliated to them, feel free to give it a try, they have a free optimizer.