Increasing Screen Size/Resolution on a VirtualBox Instance of Centos

A maximum resolution of 800x600 suggests that your X server inside the virtual machine is using the SVGA driver. SVGA is the highest resolution for which there is standard support; beyond that, you need a driver.

VirtualBox emulates a graphics adapter that is specific to VirtualBox, it does not emulate a previously existing hardware component like most other subsystems. The guest additions include a driver for that adapter. Insert the guest additions CD from the VirtualBox device menu, then run the installation program. Log out, restart the X server (send Ctrl+Alt+Backspace from the VirtualBox menu), and you should have a screen resolution that matches your VirtualBox window. If you find that you still need manual tweaking of your xorg.conf, the manual has some pointers.

There's a limit to how high you can get, due to the amount of memory you've allocated to the graphics adapter in the VirtualBox configuration. 8MB will give you up to 1600x1200 in 32 colors. Going beyond that is mostly useful if you use 3D.

I had the same problems with different setup:

Host OS: Linux Mint 17 Cinnamon 64-bit Guest OS: Centos 6.6 Virtualbox: Oracle VM VirtualBox Manager 4.3.10_Ubuntu

Solution which worked for me:

1. Install Virtualbox --> Devices, Insert Guest Additions CD image

   [root@centos6 ~]# cd /media/VBOXADDITIONS_4.3.10_93012/
   [root@centos6 ~]# ./VBoxLinuxAdditions.run
   

2. get custom resolution string:

   [root@centos6 ~]$ cvt 1600 900
   # 1600x900 59.95 Hz (CVT 1.44M9) hsync: 55.99 kHz; pclk: 118.25 MHz Modeline "1600x900_60.00"  118.25  1600 1696 1856 2112  900 903 908 934 -hsync +vsync
   

3. set custom resolution string:

   [root@centos6 ~]$ xrandr
   Screen 0: minimum 64 x 64, current 1024 x 768, maximum 16384 x 16384
   VBOX0 connected 1024x768+0+0 0mm x 0mm
      1024x768       60.0*+   60.0*
      1600x1200      60.0 
      1440x1050      60.0 
      1280x960       60.0 
      800x600        60.0 
      640x480        60.0 
   
   [root@centos6 ~]$ xrandr --newmode "1600x900_60.00" 118.25  1600 1696 1856 2112  900 903 908 934 -hsync +vsync
   
   [root@centos6 ~]$ xrandr --addmode VBOX0 "1600x900_60.00"
   

4. use normal system administration config dialog to change resolution to 1600 x 900 which was not available before.