The property MemorySizeMiB was set to ReadOnly as we do not want to change the size of the LD after it has been created/bound to a host. That would cause reprogramming of the decoders and difficulty for the host when the memory size is changed underneath. The size is set when the LD is created.
To expand a bit more, since this value is set during the creation of the logical device, if the FM needs to change the size, it would need to remove the logical device, then create a new logical device with the desired size. At this time, we don't see a need to allow for changing the size on-the-fly based on the current use cases.