Unless you specified otherwise in the tablespace definition; each page
of a table will occupy 4k of disk space. Multiply the number of pages by
4k and you have the physical space occupied. This space also includes
freespace on the pages.

The number of leaf pages for an index contains a count of all except 1
page for two level indexes. For three level indexes; you can make a
guess by multiplying the number of leaf pages by a number between 1.0
and 1.2. This is a very crude technique.

Sum up the number of pages for all of the indexes, add the space
occupied by the data and you'll have your final answer. LOB obkects,
stored separately from other table data, occupy additional space.

Any technique that uses ...pages counts in the catalog is accurate only
immediately after running runstats. If the statistics are stale, then
the size estimates will be stale.

There are also other ways of determining space.

Philip Sherman