1) if you have control center on your work station then right click on the table and click on the estimate size option.this will show u the size of the table and indexes separately in Mb or Kb.
2) You can check the table size by running a REORGCHK with update statistics. then one of the values displayed for each column is TSIZE that is the table size so just run runstats for the desire table.
## reorgchk update statistics on table schemaname.urtabname
Remember to update your statistics before you select any value from the syscat tables.
If you check the documentation for REORGCHK it will tell you the formula to calculate TSIZE from syscat tables.