I was able to set the resolution with a bootparameter in /etc/default/grub but when the tty loads, it gets overriden by something.
Then I found this: https://github.com/MichaIng/DietPi/issues/1227#issuecomment-368846000 and I added / changed this:
(sudo nano /etc/default/grub)
GRUB_CMDLINE_LINUX_DEFAULT="nomodeset"
GRUB_GFXMODE=800x600
GRUB_GFXPAYLOAD_LINUX=keep
Change GRUB_GFXMODE= to your desired resolution.
then do: sudo update-grub and reboot.
It worked for me 
edit: Here a guide how to find out working resolutions for your TV:
https://feitam.es/how-to-change-display-resolution-in-tty-console/