How can I find out the space used on disk for a table/index

Hi,

I want to judge the performance gain of additional indexes against the
space they consume on disk.  Currently I do a VACUUM FULL ANALYZE and
then take the space used by the base directory and do some
calculations.  This is time consuming, error-prone and unreliable.

Is there a quick way to compute the space used on disk by an index (in
MB or blocks) using the statistics tables?

Thanks,
Viktor