Problem is simple, in the options menu some players, including myself have very limited options for the resolutions. In my case I have only these 2 options ( https://imgur.com/a/9PyKMhc ). This problem also exists in the 0.62 build. The main problem is that being unable to change the full-screen resolution impacts performance substantially.
Here are other reports that I have found regarding this issue:
https://steamcommunity.com/app/221100/discussions/0/1734333281940202724/
https://steamcommunity.com/app/221100/discussions/1/2579854400755930580/
https://www.reddit.com/r/dayz/comments/5hrv02/cant_change_resolution/