After fresh install of guest OSs (Ubuntu, Mint linux and etc….), window resolution set by maximum 800×600.
Here is how to change that.
1. “Install Guest Additions”
Click “Install Guest Additions” on top of menu under Device tap
2. You can see below icon on your backgroud
3.Click right button of mouse and choose “Open With Autorun Prompt”
4. Click Run to complete installation
5. Once the installation is done, recompile the guest kernel modules by using the following command:
Linux_Server#
/etc/init.d/vboxadd setup
6. Reboot your guest OS.
7. Now you should be able to adjust window resolution size.