Virtual Box supports up to 256 MB of video RAM. This can not be set using the slider of the Virtual Box Manager.
To make full use of all supported memory we can issue the following command in a terminal:
$ VBoxManage modifyvm "Name of VM" --vram 256
Before we change settings such as the video RAM a pre-existing virtual machine has to be shut down.
Note that for 3D video hardware acceleration from the guest addition’s video driver the physical RAM of the host graphics card will be passed through. The video RAM settings of the virtual machine will not affect this.