Calm down.
Dead Rising 3 has some glaring PC port issues, this being one of them. There doesn't seem to be a confirmed way to set a resolution manually yet. The devs may patch it in later, hopefully they do.
One thing you can try that may or may not work and is completely at your own risk (I'm literally pulling this out of my ass):
Back up viewport.txt in steam\SteamApps\common\Dead Rising 3\data\datafile\ to any folder you want, but do back it up.
Then open the original viewport.txt in the datafile folder and look for:
cViewportShape MainView_16x9
Under there is a value for Y and a value for X. Change the value for Y to 1366 and X to 768. Like this:
cViewportShape MainView_16x9
{
X = "1366"
Y = "768"
Width = "1"
Height = "1"
Aspect="1.777777778"
}
If that section isn't in your viewport.txt at all just copy it from here and add it.
Save and exit, and launch the game.
If it doesn't work just restore the backup viewport.txt.