You can increase the boeard size in options. Retina display is nothing special, its just higher res. I have a PC with 1920x1080 (laptop) and a PC screen with 2560x1600. Its the same problem with all websites, they are made for smaller resolutions. Get used to it.
It would be good to get the chess.com application updated to include nice, sharp retina graphics as chess pieces etc. look quite blocky at the moment.
I know, I know, firstworldproblem ;)
If someone at chess.com could let me/us know if this is in the works, hopefully it shouldn't be too much work to implement the extra assets.
Thanks,
-- Dan.