I have an ultra-wide LG 34UM95-P monitor with a 21:9 ratio and 3440x1440 resolution. The game would not launch full screen at all - it only runs windowed (the option is checked and greyed out) with other settings selected as 1920x1200, 16:10 mode. It starts up at the left corner of the screen and I can't even reposition it.
Most new games support ultra-wide screens: I played GTA V with tons of joy and frankly quite surprised to see this. Fallout NV had no problem to run full screen either, but with vertical bars. Even that is better than windowed, but I was expecting support for 21:9 aspect ration in all modern games. Anyone else having the same issue?

Specs: Windows 10, Intel i5 @ 3.5 GHz, GeForce GTX 970, 8 GB RAM, LG 34UM95-P Monitor

I went to C:\Users\tj\Documents\My Games\Fallout4 and edited the Fallout4Prefs.ini and just typed in 3440 and 1440 for the resolution. The loading menu was at least centered with black borders, and the game itself is playing fullscreen. Give it a try.

Same Issue here. Same Monitor. It's the games fault. If you want to play it with bars, you can change the monitor's resolution in Windows to 1920x1080. It's sh** but works until we get a patch (haha...) or a mod (more likely).