Graphical desktops (for example Mageia) usually run faster with more video memory, however VBox application (through setting menu) allows setup the maximal size of video mamory to 128MB only. We are able add more memory using command line. The maximal value is now 256MB.
Windows (tested under win7/10 64bit)
1) go to the VBox install directory and start win command line there
2) run command
VboxManage.exe showvminfo VBOX-MACHINE_NAME
where VBOX Machine is Mageia in my case. we should be able see actual video memory size of 128MB
c:\Program Files\Oracle\VirtualBox>VBoxManage.exe showvminfo Mageia
Name: Mageia
Groups: /
Guest OS: Mandriva
UUID: 25081536-4cdd-43ff-8be0-a0f7dac73e1c
Config file: C:\Users\palo.KIS\VirtualBox VMs\Mageia\Mageia.vbox
Snapshot folder: C:\Users\palo.KIS\VirtualBox VMs\Mageia\Snapshots
Log folder: C:\Users\palo.KIS\VirtualBox VMs\Mageia\Logs
Hardware UUID: 25081536-4cdd-43ff-8be0-a0f7dac73e1c
Memory size: 512MB
Page Fusion: off
VRAM size: 128MB
CPU exec cap: 100%
HPET: off
Chipset: piix3
Firmware: BIOS
Number of CPUs: 1
Synthetic Cpu: off
CPUID overrides: None
3) Assign more memory running
VBoxManage.exe modifyvm VBOX-MACHINE_NAME --vram NEW_MEM_SIZE
in my case
c:\Program Files\Oracle\VirtualBox>VBoxManage.exe modifyvm Mageia --vram 256
4) finally check if it was modified
c:\Program Files\Oracle\VirtualBox>VBoxManage.exe showvminfo Mageia
Name: Mageia
Groups: /
Guest OS: Mandriva
UUID: 25081536-4cdd-43ff-8be0-a0f7dac73e1c
Config file: C:\Users\palo.KIS\VirtualBox VMs\Mageia\Mageia.vbox
Snapshot folder: C:\Users\palo.KIS\VirtualBox VMs\Mageia\Snapshots
Log folder: C:\Users\palo.KIS\VirtualBox VMs\Mageia\Logs
Hardware UUID: 25081536-4cdd-43ff-8be0-a0f7dac73e1c
Memory size: 512MB
Page Fusion: off
VRAM size: 256MB
CPU exec cap: 100%
Linux
And the same for Linux
<~> $ vbm showvminfo VBOX-MACHINE_NAME | grep VRAM VRAM size: 128MB <~> $ vbm modifyvm VBOX-MACHINE_NAME --vram 256 <~> $ vbm showvminfo VBOX-MACHINE_NAME | grep VRAM VRAM size: 256MB