I've recently reformatted my PC and installed Don't Starve Together back on it as well. My game does not seem to go full screen mode.
I've already tried changing the settings within the interface options. I have also tried to edit the .ini file to manually set my settings to full screen. No luck there as well.
Is anyone else having this issue? I tried looking into a fix for this from past issues on older OS's but I have been unsuccessful.
Load the game regularly.