Look for vramMultiplier in osinfo-defaults.properties file.
The following formula applies: vram_size = vramMultiplier * vgamem
You must restart Engine to apply the new setting.
The only thin I found about it in that file is :
os.rhel_7x64.devices.display.vramMultiplier.value = 2
I am not sure this file is what I want : from what it seems to only affects some
parameters at the creation of the OS.
i.e. if I take an ubuntu but I set it up as RHEL7, it won't have more vram.
With centOS7 however (that I have set as RHEL7 at its creation), it has more vram, but not
2*, really more :
centOS has : "vram_size=33554432" from what qemu says
and all others VM have 8 instead of...33