Unless you configure the replication handler, the on-disk size of each core's index seems to be unavailable in the gui or from the mbeans handler. If you are not doing replication, you should still be able to get the size of each index without configuring things that won't be used.
Also, I would like to get the size of the index in a consistent unit of measurement, probably MB. I understand the desire to give people a human readable unit next to a number that's not enormous, but it's difficult to do programmatic comparisons between values such as 787.33 MB and 23.56 GB. That may mean that the number needs to be available twice, one format to be shown in the admin GUI and both formats available from the mbeans handler, for scripting.