- Go to C:\Users\xxx\Documents\DayZ\
- Open DayZ.cfg with notepad (or any text editor)
- Replace everything with the below (make back-ups)

language="English";
adapter=-1;
3D_Performance=1500000;
Resolution_Bpp=32;
Windowed=1;
Resolution_W=1280;
Resolution_H=720;
winX=50;
winY=50;
winW=1280;
winH=720;
winDefW=1280;
winDefH=720;
Render_W=1280;
Render_H=720;
FSAA=0;
postFX=0;
GPU_MaxFramesAhead=1000;
GPU_DetectedFramesAhead=3;
HDRPrecision=8;
lastDeviceId="";
localVRAM=2087387136;
nonlocalVRAM=2147483647;
vsync=0;
AToC=0;
SWLayersCount=0;
PipQuality=3;
PPAA=0;
ssaoEnabled=0;

This will disable everything or set it as low as possible, and makes the game run in a 1280x720 window. After the game loads, go to Configure > Video > User Interface... and choose your resolution or simply resize the window before adjusting any other settings.

If you absolutely don't want to change the rest of your graphical settings, just modify these lines (should work fine, but try the above if it doesn't):
Windowed=1;
Resolution_W=1280;
Resolution_H=720;
winDefW=1280;
winDefH=720;
Render_W=1280;
Render_H=720;
Resolution = Overall resolution the game will run at.
winDef = Interface resolution, includes menus and text.
Render = Resolution the game will be rendered at before up/downscaling to overall resolution.

Note: If your character/game looks "wide," go into Configure > Video > User Interface and make sure your Aspect Ratio is properly set. 1600x900 and 1920x1080 are 16:9 resolutions, for example.