I'm using an Ubuntu emulator (VMware) on a Macbook.
My Mac says 3 GB free, while my Ubuntu emulator says 16 GB free.
I am wondering if my Ubuntu installation is "reserving" those 16gb, or it is just a wrong parameter?
In Fusion, a virtual disks can be pre-allocated or not.
If you pre-allocate a 20GB disk (the default, I think) for your virtual machine, then Fusion immediately reserves 20GB on your physical disk by creating 20GB worth of VMDK files (these files hold the virtual disk information). Your host (the mac) cannot use that space until you delete the virtual machine and it's virtual disk files.
If you chose non-pre-allocated virtual disks for your virtual machine, then Fusion doesn't reserve the 20GB of space, instead it creates a bunch of VMDK files that will act like balloons. The ballons will fill up as your Ubuntu virtual machine writes to the virtual disk. In this case, it is possible that your virtual machine will think it has 20GB of space, but the Mac really doesn't have that much left and write operation in your virtual machine may fail.
It is important to remember that in both cases, the Ubuntu virtual machine will think it has a 20GB disk attached. The VM doesn't see anything different regardless of pre-allocated or not.
What the Ubuntu virtual machine sees is a virtual disk. The virtual disk only takes up as much space as is needed (plus a little bit), so it is limited by the physical disk. You can, however, set the size of the virtual disk to be as much as VMware will support, regardless of the host disk size. The virtual machine will think the virtual disk is as big as you specify.
Whether you can fill the virtual disk will depend on the physical hard drive, however.