一般Virtualbox安装centos安装完成后，默认的分辨率是800x600。
解决：
安装Guest additions。
安装完成后，分辨率可以修改为1024x768，还有一些更高的分辨率，但是没有1366x768。
修改/etc/X11/xorg.config
安装完Guest additions后，文件默认内容为(或者类似)
# VirtualBox generated configuration file
# based on /etc/X11/xorg.conf.
Section "Monitor"
Identifier "Monitor[0]"
ModelName "VirtualBox Virtual Output"
VendorName "Oracle Corporation"
EndSection
Section "Device"
BoardName "VirtualBox Graphics"
Driver "vboxvideo"
Identifier "Device[0]"
VendorName "Oracle Corporation"
EndSection
Section "Screen"
SubSection "Display"
Depth 24
EndSubSection
Device "Device[0]"
Identifier "Screen[0]"
修改Section "Screen"，添加字段Modes "800x600" "1366x768"
Section "Screen"
SubSection "Display"
Depth 24
Modes "800x600" "1366x768"
EndSubSection
Device "Device[0]"
Identifier "Screen[0]"
注意：Modes要添加在SubSection之间，而且不能只添加1366x768，要同时添加800x600。