Looks like "game" is back, right near the bottom, just above "Screensaver attributes"
I would like solid definitive correct sizes for things as well:
When I set mine to 290x218, it looks PIXEL PERFECT on my Roku 4 and on my 4k Roku TV where the UI is 1080p.
Maybe the 540x405 is for the larger channel picture that you see when surfing the channel store?
Komag and Marcelo, I've confirmed the dimensions listed for the manifest attributes in the documentation is accurate. However, please be aware that these assets are only used when your channel is side-loaded. When your channel is published and installed from the Channel Store, the channel icon that renders is the one that you upload during the channel creation/submission flow.
540x405 is the dimension for the asset uploaded during channel submission. This is the pixel perfect spec, and scaling is handled by the firmware.
If this is incongruous with your experience, can you shine a little more light on the specific use cases where things are going array?