If I create a new avd using the manager, it does not matter if I set the disk.dataPartition.size property, when I start the emulator the partition size will be the default!
It happens, because the ini file generated contains this: disk.dataPartition.size=8192 MB
However, if I edit the ini file before starting the emulator, and correct the line as: disk.dataPartition.size=8192M, it finally works as intended!
Please, look after this issue!