I too cannot get chromcast 2 to display in full screen. seems to be only in 4:3 aspect ratio.
i have connected my satellite box to slingplayer 500 via component.
my satellite box is outputing to the TV in 16:9 and i have tried all settings on the satellite box to try and stretch the image, this has no affect on the slingplayer to chromecast 2.
the tv connected to HDMI out on the slingbox 500 is outputting perfectly at 16:9.
the tv the chromecast 2 is plugged into displays the chromcast and all other content i.e stan you tube etc in full screen, however when i sling the content from my iphone to chromcast, the slingplayer loading screen is full screen 16:9 but once the cable/tv content loads it is only in 4:3 or not filling the screen.
from reading other forums i think this maybe a bug with slingplayer but not sure.
i have latest updates on all devices.
any answer on this would be much appreciated.