Well that's what I'm trying to figure out. From what I understand, the proper fix for this is for the OB headunit manufacture to update their firmware to "inform" Android Auto at runtime the size of the screen real estate available, and then Android Auto would be able to take advantage of the full screen available. I'm assuming that will happen sometime in the next few months.
But I noticed yesterday that the AAWireless dongle has this ability to manual override the DPI setting that gets sent to Android Auto. I don't quite know if this is the exact same setting that the headunit manufacturer is supposed to update as well, so I was just wondering if anyone had tried it, and if it worked.
When I finally get my AAWireless unit and try this for myself, I will post my results here.