There are many posts here about people trying to set a custom resolution but they all have similar answers;
Uncomment and set the following two values to your preference; hdmi_group=2 hdmi_mode=39
So in my case I want 1360x768 which is value 39 above.
My problem is that I have tried all different values listed at http://elinux.org/RPi_config.txt. Also with hdmi_group=1. It never works. Everytime I reboot my Pi or Pi and TV, it always starts up in 720P which doesn't fit the screen properly. I want 1360x768.
I have also tried setting hdmi_safe=1, whilst this squashed the picture a bit, it didn't really help.